Low profile spinal prosthesis incorporating a bone anchor having a deflectable post and a compound spinal rod

ABSTRACT

A bone anchor comprising a self-centering ball-joint suitable for use in a spine stabilization prosthesis which supports the spine while providing for the preservation of spinal motion. The bone anchor has a deflectable ball-rod partially received in a socket of a housing formed in the head of the bone anchor. A centering rod received partially in the ball-rod and partially within the housing operates to align the ball-rod with the longitudinal axis of the bone anchor. Deflection of the ball-rod bends the centering rod which in turn applies a restoring force upon the ball-rod.

CLAIM TO PRIORITY

This patent application claims priority to the following patents andpatent applications:

U.S. patent application Ser. No. 12/629,811, filed Dec. 2, 2009,entitled “Low Profile Spinal Prosthesis Incorporating A Bone AnchorHaving A Deflectable Post And A Compound Spinal Rod”; and

International Patent Application No. PCT/US2009/066567, filed Dec. 3,3009, entitled “Low Profile Spinal Prosthesis Incorporating A BoneAnchor Having A Deflectable Post And A Compound Spinal Rod”.

All of the afore-mentioned patent applications are incorporated hereinby reference in their entireties.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to all of the afore-mentioned patentapplications. This application is also related to all of the followingapplications including:

U.S. patent application Ser. No. 12/566,478, filed Sep. 24, 2009,entitled “Modular In-Line Deflection Rod And Bone Anchor System AndMethod For Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,485, filed Sep. 24, 2009,entitled “Versatile Polyaxial Connector Assembly And Method For DynamicStabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,487, filed Sep. 24, 2009,entitled “Versatile Offset Polyaxial Connector And Method For DynamicStabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,494, filed Sep. 24, 2009,entitled “Load-Sharing Component Having A Deflectable Post And MethodFor Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,498, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Durable Compliant Member AndMethod For Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,504, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post With ACompliant Ring And Method For Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,507, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post With ACompliant Ring And Method For Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,511, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post And MethodFor Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,516, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Natural Center Of RotationAnd Method For Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,519, filed Sep. 24, 2009,entitled “Dynamic Spinal Rod And Method For Dynamic Stabilization Of TheSpine”; and

U.S. patent application Ser. No. 12/566,522, filed Sep. 24, 2009,entitled “Dynamic Spinal Rod Assembly And Method For DynamicStabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,529, filed Sep. 24, 2009,entitled “Configurable Dynamic Spinal Rod And Method For DynamicStabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,531, filed Sep. 24, 2009,entitled “A Spinal Prosthesis Having A Three Bar Linkage For MotionPreservation And Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,534, filed Sep. 24, 2009,entitled “Surgical Tool And Method For Implantation of A Dynamic BoneAnchor”; and

U.S. patent application Ser. No. 12/566,547, filed Sep. 24, 2009,entitled “Surgical Tool And Method For Connecting A Dynamic Bone Anchorand Dynamic Vertical Rod”; and

U.S. patent application Ser. No. 12/566,551, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post AndCentering Spring And Method For Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,553, filed Sep. 24, 2009,entitled “Load-Sharing Component Having A Deflectable Post And CenteringSpring And Method For Dynamic Stabilization Of The Spine”; and

U.S. patent application Ser. No. 12/566,559, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post And AxialSpring And Method For Dynamic Stabilization Of The Spine”.

All of the afore-mentioned patent applications are incorporated hereinby reference in their entireties.

BACKGROUND OF INVENTION

Back pain is a significant clinical problem and the costs to treat it,both surgical and medical, are estimated to be over $2 billion per year.One method for treating a broad range of degenerative spinal disordersis spinal fusion. Implantable medical devices designed to fuse vertebraeof the spine to treat have developed rapidly over the last decade.However, spinal fusion has several disadvantages including reduced rangeof motion and accelerated degenerative changes adjacent the fusedvertebrae.

Alternative devices and treatments have been developed for treatingdegenerative spinal disorders while preserving motion. These devices andtreatments offer the possibility of treating degenerative spinaldisorders without the disadvantages of spinal fusion. However, currentdevices and treatments suffer from disadvantages e.g., complicatedimplantation procedures; lack of flexibility to conform to diversepatient anatomy; the need to remove tissue and bone for implantation;increased stress on spinal anatomy; insecure anchor systems; poordurability, and poor revision options. Consequently, there is a need fornew and improved devices and methods for treating degenerative spinaldisorders while preserving motion.

SUMMARY OF INVENTION

The present invention includes a spinal implant system and methods thatcan dynamically stabilize the spine while providing for the preservationof spinal motion. Embodiments of the invention provide a dynamicstabilization system which includes: versatile components, adaptablestabilization assemblies, and methods of implantation. An aspect ofembodiments of the invention is the ability to stabilize two, threeand/or more levels of the spine by the selection of appropriatecomponents of embodiments of the invention for implantation in apatient. Another aspect of embodiments of the invention is the abilityto accommodate particular anatomy of the patient by providing a systemof versatile components which may be customized to the anatomy and needsof a particular patient and procedure. Another aspect of the inventionis to facilitate the process of implantation and minimize disruption oftissues during implantation.

Thus, the present invention provides new and improved systems, devicesand methods for treating degenerative spinal disorders by providing andimplanting a dynamic spinal stabilization assembly which supports thespine while preserving motion. These and other objects, features andadvantages of the invention will be apparent from the drawings anddetailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective views of a deflection system componentmounted to an anchor system component according to an embodiment of thepresent invention.

FIG. 1C is a perspective view of a connection system component mountedto an anchor system component according to an embodiment of the presentinvention.

FIG. 1D is a perspective view of a different connection system componentmounted to an anchor system component according to an embodiment of thepresent invention.

FIG. 1E is a posterior view of an anchor system for a multi-leveldynamic stabilization assembly utilizing the anchor components of FIGS.1A to 1D according to an embodiment of the present invention.

FIG. 1F is a posterior view of a multi-level dynamic stabilizationassembly utilizing the components of FIGS. 1A to 1E according to anembodiment of the present invention.

FIG. 2A is an exploded view of a deflection rod according to anembodiment of the present invention.

FIG. 2B is a perspective view of the deflection rod assembly of FIG. 2A,as assembled.

FIG. 2C is a sectional view of the deflection rod assembly of FIGS. 2Aand 2B.

FIG. 2D is a sectional view of the deflection rod assembly of FIGS. 2Aand 2B.

FIGS. 2E and 2F are sectional views of the deflection rod assembly ofFIGS. 2A and 2B showing deflection of the post.

FIG. 2G is a transverse sectional view of a vertebra illustrating theimplantation of the deflection rod assembly of FIGS. 2A and 2B.

FIG. 3A is an exploded view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 3B is a perspective view of the deflection rod assembly of FIG. 3A,as assembled.

FIG. 3C is a sectional view of the deflection rod assembly of FIGS. 3Aand 3B.

FIG. 3D is a sectional view of the deflection rod assembly of FIGS. 3Aand 3B showing deflection of the post.

FIG. 3E is a transverse sectional view of a vertebra illustrating theimplantation of the deflection rod assembly of FIGS. 3A and 3B.

FIG. 3F is a transverse sectional view of a vertebra illustrating theimplantation of an alternative deflection rod.

FIG. 3G is a lateral view of a multi-level dynamic stabilizationassembly utilizing the deflection rod assembly of FIGS. 3A-3B accordingto an embodiment of the present invention.

FIG. 3H is an oblique view of an offset connector mounted to thedeflection rod assembly of FIGS. 3A-3B according to an embodiment of thepresent invention.

FIG. 3I shows a socket with interior features adapted to engage featuresof the housing of a deflection rod assembly according to an embodimentof the present invention.

FIG. 3J shows a connector with interior features adapted to engagefeatures of the housing of a deflection rod assembly according to anembodiment of the present invention.

FIG. 4A shows an exploded view of an alternative bone anchor accordingto an embodiment of the invention.

FIG. 4B shows a perspective view of the alternative bone anchor of FIG.4A as assembled.

FIG. 4C shows a sectional view of the alternative bone anchor of FIG. 4Aas assembled.

FIG. 5A shows an exploded view of an alternative spinal rod according toan embodiment of the invention.

FIG. 5B shows a perspective view of the alternative spinal rod of FIG.5A as assembled.

FIG. 5C shows a sectional view of the alternative spinal rod of FIG. 5Aas assembled.

FIG. 5D shows a lateral of a spinal prosthesis incorporating the boneanchor of FIGS. 4A-4C and spinal rod of FIGS. 5A-5C according to anembodiment of the present invention.

FIG. 6A illustrates an aspect of the kinematics of a spinal prosthesisincorporating the bone anchor of FIGS. 4A-4C and spinal rod of FIGS.5A-5C according to an embodiment of the present invention.

FIG. 6B illustrates an aspect of the kinematics of the spinal prosthesisof FIG. 6A.

FIG. 6C illustrates an aspect of the kinematics of the spinal prosthesisof FIG. 6A.

FIGS. 7A-7C show views of an alternative vertical rod.

FIG. 8A shows an exploded view of an alternative bone anchor accordingto an embodiment of the invention.

FIG. 8B shows a perspective view of the alternative bone anchor of FIG.8A as assembled.

FIG. 8C shows a sectional view of the alternative bone anchor of FIG. 8Aas assembled.

FIG. 8D shows a sectional view of the alternative bone anchor of FIG. 8Aillustrating deflection of the deflectable post.

FIG. 9A shows an exploded view of an alternative bone anchor accordingto an embodiment of the invention.

FIG. 9B shows a perspective view of the alternative bone anchor of FIG.9A as assembled.

FIG. 9C shows a sectional view of the alternative bone anchor of FIG.9A.

FIG. 9D shows a sectional view of the alternative bone anchor of FIG. 9Aas assembled and illustrating deflection of the deflectable post.

FIG. 9E shows a sectional view of a variation of the alternative boneanchor of FIG. 9A as assembled and illustrating deflection of thedeflectable post.

FIG. 10A shows a schematic view of a spinal implant component utilizinga self-centering ball-joint according to an embodiment of the invention.

FIG. 10B shows a schematic view of the spinal implant component of FIG.10A illustrating deflection of the ball-joint.

FIG. 10C shows a schematic view of an alternative spinal implantcomponent utilizing a self-centering ball-joint according to anembodiment of the invention.

FIGS. 11A-11F show alternative embodiments for centering rods for use inembodiments of the present invention.

FIG. 12A shows an exploded view of a compound spinal rod according to anembodiment of the present invention.

FIG. 12B shows a perspective view of the compound spinal rod of FIG. 12Aas assembled.

FIG. 12C shows a sectional view of the compound spinal rod of FIG. 12Aas assembled.

FIG. 12D shows a view of spinal implant prosthesis utilizing thecompound spinal rod of FIGS. 12A-12C in conjunction with the bone anchorof FIGS. 9A-9D.

FIG. 12E shows a partial sectional view of a spinal implant prosthesisutilizing the compound spinal rod of FIGS. 12A-12C in conjunction withthe bone anchor of FIGS. 9A-9D.

FIG. 13A shows a perspective view of an implantation tool for a dynamicbone anchor according to an embodiment of the invention.

FIGS. 13B and 13C show detailed sectional views of the head of theimplantation tool of FIG. 13A in relation to a dynamic bone anchor.

FIG. 13D is a transverse view of the lumbar spine illustrating use ofthe implantation tool of FIG. 13A to implant a dynamic bone anchor inthe pedicles of a lumbar vertebra according to an embodiment of theinvention.

FIG. 14A shows a perspective view of an attachment tool for securing adynamic spinal rod to a dynamic bone anchor according to an embodimentof the invention.

FIG. 14B shows a detailed view of the head of the attachment tool ofFIG. 14A.

FIGS. 14C and 14D show detailed sectional views of the head of theattachment tool of FIG. 14A in relation to a dynamic spinal rod and boneanchor.

FIG. 14E-14H are a lateral views of the lumbar spine illustrating stepsto secure a dynamic spinal rod to a dynamic bone anchor assembly usingthe attachment tool of FIG. 14A according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes a versatile spinal implant system andmethods which can dynamically stabilize the spine while providing forthe preservation of spinal motion. Alternative embodiments can be usedfor spinal fusion. An aspect of the invention is restoring and/orpreserving the natural motion of the spine including the quality ofmotion as well as the range of motion. Still, another aspect of theinvention is providing for load sharing and stabilization of the spinewhile preserving motion.

Another aspect of the invention is to provide a modular system which canbe customized to the needs of the patient. Another aspect of embodimentsof the invention is the ability to stabilize two, three and/or morelevels of the spine by the selection of appropriate components forimplantation in a patient. Another aspect of the invention is theability to provide for higher stiffness and fusion at one level or toone portion of the spine while allowing for lower stiffness and dynamicstabilization at another adjacent level or to another portion of thespine. Embodiments of the invention allow for fused levels to be placednext to dynamically stabilized levels. Such embodiments of the inventionenable vertebral levels adjacent to fusion levels to be shielded byproviding a transition from a rigid fusion level to a dynamicallystable, motion preserved, and more mobile level.

Embodiments of the present invention provide for assembly of a dynamicstabilization system which supports the spine while providing for thepreservation of spinal motion. The dynamic stabilization system has ananchor system, a deflection system, a vertical rod system and aconnection system. The anchor system anchors the construct to the spinalanatomy. The deflection system provides dynamic stabilization whilereducing the stress exerted upon the bone anchors and spinal anatomy.The vertical rod system connects different levels of the construct in amultilevel assembly and may in some embodiments include compounddeflection rods. The connection system includes coaxial connectors andoffset connectors which adjustably connect the deflection system,vertical rod system and anchor system allowing for appropriate,efficient and convenient placement of the anchor system relative to thespine. Alternative embodiments can be used for spinal fusion.

Embodiments of the invention include a construct with an anchor system,a deflection system, a vertical rod system and a connection system. Thedeflection system provides dynamic stabilization while reducing thestress exerted upon the bone anchors and spinal anatomy. The anchorsystem anchors the deflection system to the spine. The connection systemconnects the deflection system to the vertical rod system. The verticalrod system connects dynamic stabilization system components on differentvertebra to provide load sharing and dynamic stabilization.

Embodiments of the present invention include a deflectable post whichprovides load sharing while preserving range of motion and reducingstress exerted upon the bone anchors and spinal anatomy. The deflectablepost is connected to a bone anchor by a ball-joint which permits thedeflectable post to pivot and rotate relative the bone anchor. Thekinematics of the deflectable post may be adapted to the anatomy andfunctional requirements of the patient.

Embodiments of the present invention include a deflectable post whichprovides load sharing while preserving range of motion and reducingstress exerted upon the bone anchors and spinal anatomy. The deflectablepost is connected to a bone anchor by a ball-joint which permits thedeflectable post to pivot and rotate relative the bone anchor. Aflexible centering rod within the ball-joint serves to align thedeflectable post with the axis of the bone anchor. The kinematics of thedeflectable post may be adapted to the anatomy and functionalrequirements of the patient.

Embodiments of the present invention include a compound spinal rod whichprovides load sharing while preserving range of motion and reducingstress exerted upon the bone anchors and spinal anatomy. The compoundspinal rod includes a coupling which is adapted to be fixed to thedeflectable post. The coupling is connected by a pivoting joint to a rodwhich is adapted to be connected to a bone anchor on an adjacentvertebra. The pivoting joint permits the spinal rod to pivot about anaxis perpendicular to the longitudinal axis of the spinal rod.

Embodiments of the present invention include a compound spinal rod whichprovides load sharing while preserving range of motion and reducingstress exerted upon the bone anchors and spinal anatomy. The compoundspinal rod includes a rod-end which is adapted to be fixed to thedeflectable post. The rod-end is connected by a sliding-rotating jointto a rod which is adapted to be connected to a bone anchor on anadjacent vertebra. The rod-end includes a coupling to mount to a boneanchor. The sliding-rotating joint permits the coupling to be positionedsuch that the deflectable post is oriented in a preferred orientationrelative to the bone anchor of which it is part.

Embodiments of the present invention include an assembly comprising abone anchor, and deflectable post assembled with a compound spinal rod.The assembly provides load sharing while preserving range of motion andreducing stress exerted upon the bone anchors and spinal anatomy. Thedeflectable post is connected to a bone anchor by a ball-joint whichpermits the deflectable post to pivot and rotate relative the boneanchor. The compound spinal rod includes a coupling which is adapted tobe fixed to the deflectable post. The coupling is connected by apivoting joint to a rod which is adapted to be connected to a boneanchor on an adjacent vertebra. The pivoting joint permits the spinalrod to pivot about an axis perpendicular to the longitudinal axis of thespinal rod. The assembly permits movement of adjacent vertebrae in amanner closely approximately the natural kinematics of the spine.

Common reference numerals are used to indicate like elements throughoutthe drawings and detailed description; therefore, reference numeralsused in a drawing may or may not be referenced in the detaileddescription specific to such drawing if the associated element isdescribed elsewhere. The first digit in a three digit reference numeralindicates the series of figures in which the referenced item firstappears. Likewise the first two digits in a four digit referencenumeral.

The terms “vertical” and “horizontal” are used throughout the detaileddescription to describe general orientation of structures relative tothe spine of a human patient that is standing. This application alsouses the terms proximal and distal in the conventional manner whendescribing the components of the spinal implant system. Thus, proximalrefers to the end or side of a device or component closest to the handoperating the device, whereas distal refers to the end or side of adevice furthest from the hand operating the device. For example, the tipof a bone screw that enters a bone would conventionally be called thedistal end (it is furthest from the surgeon) while the head of the screwwould be termed the proximal end (it is closest to the surgeon).

Dynamic Stabilization System

FIGS. 1A-1F introduce components of a dynamic stabilization systemaccording to an embodiment of the present invention. The componentsinclude anchor system components, deflection rods, vertical/spinal rodsand connection system components, including for example coaxial andoffset connectors. The components may be implanted and assembled to forma dynamic stabilization system appropriate for the anatomical andfunctional needs of a patient.

FIG. 1A shows a bone anchor 102 and a deflection rod 104 connected to avertical/spinal rod 106 by a ball joint 108. Deflection rod 104 is anexample of a component of the deflection rod assembly system. Deflectionrod 104 is a component having controlled flexibility which allows forload sharing. The deflection rod 104 provides stiffness and supportwhere needed to support the loads exerted on the spine during normalspine motion, which loads, the soft tissues of the spine are no longerable to accommodate since these spine tissues are either degenerated ordamaged. Load sharing is enhanced by the ability to select theappropriate stiffness of the deflection rod in order to match the loadsharing characteristics desired. For embodiments of this invention, theterms “deflection rod” and “loading rod” can be used interchangeably.Deflection rods, deflection rod mountings and alternative deflectionrods are described in more detail below.

Deflection rod 104 includes a deflectable post 105 which may deflectrelative to a mount 107. Mount 107 is adapted to secure the deflectablepost 105 to bone anchor 102. Mount 107 is received within cavity 132 ofbone anchor 102. When received in cavity 132, mount 107 is secured intoa fixed position relative to bone anchor 102. Deflectable post 105 maystill deflect in a controlled manner relative to bone anchor 102 therebyprovide for load sharing while preserving range of motion of thepatient. The stiffness/flexibility of deflection of the deflectable post105 relative to mount 107/bone anchor 102 may be controlled and/orcustomized as will be described below.

As shown in FIG. 1A, mount 107 is designed to be received within acavity 132 of bone anchor 102. As shown in FIG. 1A, mount 107 includes acollar 140. A threaded aperture 142 extends obliquely through collar140. The threaded aperture 142 receives a locking set screw 144 which,when seated (FIG. 1B), engages the housing 130 of bone anchor 102.Locking set screw 144 is positioned within threaded aperture 142 throughcollar 140. The locking set screw 144 thereby secures the mount 107 ofdeflection rod 104 in place within the housing 130 of bone anchor 102.

As shown in FIG. 1A, deflection rod 104 is oriented in a co-axial,collinear or parallel orientation to bone anchor 102. This arrangementsimplifies implantation, reduces trauma to structures surrounding animplantation site, and reduces system complexity. Arranging thedeflection rod 104, co-axial with the bone anchor 102 can substantiallytransfer a moment (of) force applied by the deflectable post 105 from amoment force tending to pivot or rotate the bone anchor 102 about theaxis of the shaft, to a moment force tending to act perpendicular to theaxis of the shaft. The deflection rod can thereby effectively resistrepositioning of the deflection rod and/or bone anchor 102 without theuse of locking screws or horizontal bars to resist rotation. Furtherexamples of coaxial deflection rods are provided below. Each of thedeflection rods described herein may be used as a component of a dynamicstabilization system.

Bone anchor 102 is an example of a component of the anchor system. Boneanchor 102 includes a bone screw 120 and housing 130. As shown in FIG.1A, bone anchor 102 is a bone screw 120 having one or more threads 124which engage a bone to secure the bone anchor 102 onto a bone. Theanchor system may include one or more alternative bone anchors known inthe art e.g. bone hooks, expanding devices, barbed devices, threadeddevices, adhesive and other devices capable of securing a component tobone instead of or in addition to bone screw 120.

As shown in FIG. 1A, bone anchor 102 includes a housing 130 at theproximal end. Housing 130 includes a cavity 132 for receiving deflectionrod 104. Cavity 132 is coaxial with threaded bone screw 120. Housing 130also comprises a groove 134 for securing deflection rod 104 withinhousing 130. As shown in FIG. 1A, groove 134 is located at the proximalend of housing 130. Groove 134 is designed to be engaged by the lockingmechanism of a component mounted within cavity 132. For example, groove134 is designed to be engaged by locking set screw 144 of deflection rod104. When deflection rod 104 has been positioned within cavity 132 ofbone anchor 102 as shown in FIG. 1B, locking set screw 144 is tightenedto engage groove 134 of housing 130 thus securing deflection rod 104within housing 130. Alternative mechanisms and techniques may be used tosecure the deflection rod to the bone anchor including for example,welding, soldering, bonding, and/or mechanical fittings includingthreads, snap-rings, locking washers, cotter pins, bayonet fittings orother mechanical joints.

Bone anchor 102 also includes a coupling 136 to which other componentsmay be mounted. As shown in FIG. 1A, coupling 136 is the externalcylindrical surface of housing 130. Housing 130 thus provides twomounting positions, one coaxial mounting position and one externalmounting position. Thus, a single bone anchor 102 can serve as themounting point for one, two or more components. A deflection rod 104 maybe coaxially mounted in the cavity 132 of the housing and one or moreadditional components may be externally mounted to the outer surface 136of the housing. For example, a component of the connection system may bemounted to the outer surface 136 of the housing—such a connector may becalled an offset head or offset connector. In some applications acomponent of the connection system may be coaxially-mounted in thecavity 132 in place of a deflection rod 104—such a connector may becalled a coaxial head or coaxial connector.

It is desirable to have a range of different connectors which arecompatible with the anchor system and deflection system. The connectorsmay have different attributes, including for example, different degreesof freedom, range of motion, and amount of offset, which attributes maybe more or less appropriate for a particular relative orientation andposition of two bone anchors and/or patient anatomy. It is desirablethat each connector be sufficiently versatile to connect a vertical rodto a bone anchor in a range of positions and orientations while beingsimple for the surgeon to adjust and secure. It is desirable to providea set of connectors which allows the dynamic stabilization system to beassembled in a manner that adapts a particular dynamic stabilizationassembly to the patient anatomy rather than adapting the patient anatomyfor implantation of the assembly (for example, by removing tissue\boneto accommodate the system). In a preferred embodiment, the set ofconnectors comprising the connection system have sufficient flexibilityto allow the dynamic stabilization system to realize a suitable dynamicstabilization assembly in all situations that will be encountered withinthe defined patient population.

In some embodiments of the present invention, a connection systemcomponent, e.g. a polyaxial connector may be mounted in the cavity 132of a bone anchor 102 to secure the bone anchor to vertical rod 106. Forexample, FIG. 1C shows coaxial head 150 which is a polyaxial connectorwhich is coaxially mounted within the cavity 132 of the housing 130 ofbone anchor 102. Coaxial head 150 is an example of a coaxial head orcoaxial connector. Bone anchor 102 is the same bone anchor previouslydescribed with respect to FIGS. 1A and 1B. Coaxial head 150 comprises arod 152 which is designed to fit within cavity 132 of housing 130.Coaxial head 150 also comprises a collar 154 and locking set screw 156.Locking set screw 156 is configured to engage groove 134 of bone anchor102 in the same way as locking set screw 144 of deflection rod 104. Rod152 and cavity 132 may in some case be circular in section (e.g.cylindrical), in which case rod 152 can rotate within cavity 132 untillocked into place by locking set screw 156. In alternative embodiments,rod 152 may be polygonal in section such that it fits in one of a fixednumber of possible positions.

Referring again to FIG. 1C, attached to rod 152 of coaxial head 150 is ayoke 164. Yoke 164 is connected to a ball 165 by a hexagonal pin 162. Asaddle 163 is also mounted to ball 165 such that saddle 163 can pivotabout two orthogonal axes relative to yoke 164. Saddle 163 has anaperture 168 through which a vertical rod may be passed. On one side ofaperture 168 is a plunger 169. On the other side of aperture 168 is alocking set screw 167. When a vertical rod 106 (not shown) is positionedwithin aperture 168 and locking set screw 167 is tightened down, thelocking set screw 167 forces the vertical rod 106 down onto the plunger169. Plunger 169 is, in turn, forced down by the vertical rod 106against ball 165. Plunger 169 engages ball 165, and ball 165 engageshexagonal pin 162, to lock saddle 163 in position relative to yoke 164and secure a rod (e.g. vertical rod 106) to saddle 163. In this way,tightening set screw 167 secures the vertical rod 106 to the coaxialhead 150 and also locks orientation of the coaxial head 150.

The ability to coaxially mount coaxial head 150 to a bone anchor 102 hasseveral advantages over a standard polyaxial bone screw in which apolyaxial connector is an integral part of the device and may not beremoved or exchanged. The bone anchor 102 is simpler to install andthere is no risk of damage to the polyaxial connector duringinstallation. A single coaxial head 150 can be manufactured and designedto mount to a range of different bone anchors thus allowing bone anchorsto be selected as appropriate for the patient anatomy. After the boneanchor is installed, the orientation of the yoke 164 can be adjustedwithout changing the screw depth (this is not possible in a standardpolyaxial bone screw without also turning the screw). After the boneanchor is implanted, one of a range of different coaxial heads may beinstalled without requiring removal of the bone anchor. Likewise, if arevision is required, the coaxial head may be exchanged for a differentcomponent without necessitating removal of the bone anchor 102.

As described above, bone anchor 102 has housing 130 which can accept onecoaxially-mounted component (e.g. a coaxial head) and oneexternally-mounted component (e.g. an offset connector). FIG. 1D shows acomponent of the connection system which may be mounted externally tohousing 130 of bone anchor 102 in conjunction with a coaxially-mountedcomponent. FIG. 1D shows a perspective view of offset connector 170mounted externally to housing 130 of bone anchor 102 in which adeflection rod 104 is coaxially mounted. Connector 170 may be termed anoffset head or offset connector.

Offset connector 170 comprises six components and allows for two degreesof freedom of orientation and two degrees of freedom of position inconnecting a vertical rod to a bone anchor. The six components of offsetconnector 170 are dowel pin 172, pivot pin 174, locking set screw 176,plunger 178, clamp ring 180 and saddle 182. Saddle 182 has a slot 184sized to receive a rod which may be a vertical rod, e.g. vertical rod106 of FIG. 1A. Locking set screw 176 is mounted at one end of slot 184such that it may be tightened to secure a rod within slot 184.

Clamp ring 180 is sized such that, when relaxed it can slide freely upand down the housing 130 of bone anchor 102 and rotate around housing130. However, when locking set screw 176 is tightened on a rod, theclamp ring 180 grips the housing and prevents the offset connector 170from moving in any direction. Saddle 182 is pivotably connected to clampring 180 by pivot pin 174. Saddle 182 can pivot about pivot pin 174.However, when locking set screw 176 is tightened on a rod, the plunger178 grips the clamp ring 180 and prevents further movement of the saddle182. In this way, operation of the single set screw 176 serves to lockthe clamp ring 180 to the housing 130 of the bone anchor 102, fix saddle182 in a fixed position relative to clamp ring 180 and secure a rodwithin the slot 184 of offset connector 170.

The above-described coaxial connector and offset connector are providedby way of example only. Alternative embodiments of coaxial heads andoffset connectors can be found in U.S. Provisional Patent ApplicationNo. 61/100,625, filed Sep. 26, 2008 entitled “Versatile AssemblyComponents And Methods For A Dynamic Spinal Stabilization System” whichis incorporated by reference. These coaxial heads and offset connectorsmay be used in conjunction with the components herein described topermit assembly of a dynamic stabilization system appropriate to thefunctional needs and anatomy of a particular patient. In addition screwshaving an integrated connector may also be utilized to anchor componentsof the dynamic stabilization system in fixed relationship to a vertebra,for example polyaxial screws.

The components of the dynamic stabilization system may be assembled andimplanted in the spine of a patient to provide a multilevel dynamicstabilization assembly which provides dynamic stabilization of the spineand load sharing. In some embodiments, the first step is implantation ofbone anchors in the vertebrae. In other embodiments, the bone anchorsmay be implanted with the deflection rod/connection component alreadyinstalled.

FIG. 1E, shows three adjacent vertebrae 191, 192 and 193. As apreliminary step, bone anchors 102 a, 102 b and 102 c have beenimplanted in the vertebrae 191, 192 and 193 on the right side of thespinous process 194 between the spinous process 194 and the transverseprocess 195. A driver is inserted into the cavity 132 a, 132 b, 132 c inorder to drive the threaded portion of each bone anchor into the bone.In preferred procedures, the bone anchor is directed so that thethreaded portion is implanted within one of the pedicles 196 angledtowards the vertebral body 197. The threaded region of each bone anchoris fully implanted in the vertebrae 191, 192 and 193. A driver mayalternatively and/or additionally engage the exterior surface of housing130 in order to implant the bone anchor. The driver may have atorque-measuring and/or torque limiting function to assist in accurateimplantation of the bone screw and avoid excess force being applied tothe vertebrae. In alternative embodiments, the bone screw mayincorporate a torque limiting element, for example a secondary headwhich breaks away when the driver torque exceeds a predetermined torquelimit. See, e.g. FIGS. 7F-7H and accompanying text.

As shown in FIG. 1E, the housings 130 a, 130 b, 130 c of each boneanchor remain partly or completely exposed above the surface of thevertebrae so that one or more of a connection system component anddeflection component can be secured to each bone anchor 102 a, 102 b and102 c. Coaxial components may be coaxially-mounted inside each ofcavities 132 a, 132 b, and 132 c. Offset heads/connectors may also beexternally-mounted to the outside surface of each of housings 130 a, 130b and 130 c. Note that bone anchors are also implanted on the left sideof the spine.

After installation of the bone anchors, the deflection systemcomponents, vertical rod systems components and connection systemcomponents may be installed and assembled. FIG. 1F shows one way toassemble deflection system components and connection system components.As shown in FIG. 1F, a coaxial head 150 is installed in bone anchor 102c. An offset connector 170 is mounted externally to the housing of boneanchor 102 b. A deflection rod 104 a is coaxially mounted in the housingof bone anchor 102 a. A deflection rod 104 b is coaxially mounted in thehousing of bone anchor 102 b. A vertical rod 106 a is connected at oneend to deflection rod 104 a by ball joint 108 a. Vertical rod 106 a isconnected at the other end by in-line connector 170 to bone anchor 102b. A second vertical rod 106 b is connected at one end to deflection rod104 b by ball joint 108 b. Vertical rod 106 b is connected at the otherend by coaxial head 150 to bone anchor 102 c.

The dynamic stabilization assembly 190 of FIG. 1E thus has a verticalrod 106 a, 106 b stabilizing each spinal level (191-192 and 192-193).Each of the vertical rods 106 a, 106 b is secured rigidly at one end toa bone anchor (102 b, 102 c). Each of the vertical rods 106 a, 106 b issecured at the other end by a ball joint 108 a, 108 b to a deflectionrod 104 a, 104 b thereby allowing for some movement and load sharing bythe dynamic stabilization assembly. Offset connector 170 and coaxialhead 150 permit assembly of dynamic stabilization assembly 190 for awide range of different patient anatomies and/or placements of boneanchors 102 a, 102 b and 102 c. An identical or similar dynamicstabilization assembly would preferably be implanted on the left side ofthe spine. It should be noted that dynamic stabilization assembly 190does not require horizontal bars or locking screws thereby reducing theexposure of tissue and/or bone to foreign bodies compared to systemswith this additional hardware. The dynamic stabilization assembly ofFIG. 1F, thereby, has a small footprint, potentially reducing the amountof displacement of tissue and/or bone, reducing trauma to tissue and/orbone during surgery. Further, the smaller footprint can reduce theamount of tissue that needs to be exposed during implantation.

Deflection Rods/Loading Rods

One feature of embodiments of the present invention is the load sharingand range of motion provided by a deflection rod. The deflection rodprovides stiffness and support where needed to support the loads exertedon the spine during normal spine motion thereby recovering improvedspine function without sacrificing all motion. The deflection rod alsoisolates the anchor systems components from forces exerted by thedynamic stabilization assembly thereby reducing stress on the boneanchors and the bone to which they are attached. Moreover, by selectingthe appropriate stiffness of the deflection rod or loading rod to matchthe physiology of the patient and the loads that the patient places onthe spine, a better outcome is realized for the patient.

The deflection rod includes a deflectable post, a compliant sleeve and amount. The deflectable post and mount are typically made ofbiocompatible metal or metals, e.g. titanium and stainless steel. Thesleeve is made of a compliant material, for example a compliant polymer.The mount secures the deflection rod to an anchoring device in a mannerwhich allows deflection of the deflectable post. The deflectable post isconfigured to connect to the vertical rod system. The deflectable postmay deflect relative to the mount by compressing the compliant materialof the sleeve. The deformation of the sleeve imparts force/deflectioncharacteristics to the deflectable post. The movement of the postrelative to the mount allows controlled movement of the bone anchor (andvertebra in which it is implanted) relative to the vertical rods therebysupporting the vertebrae to which the bone anchors are attached whileallowing movement of the vertebrae.

Deflection rods can be manufactured in a range from highly rigidconfigurations to very flexible configurations by appropriate selectionof the design, materials and dimensions of the post, sleeve and mount.Deflection rods having a particular stiffness/flexibility may beselected for use in a dynamic stabilization assembly based upon thephysiological needs of a particular patient. In a preferred embodimentdeflection rod stiffness/flexibility is selected to provide load sharingin conjunction with from 50% to 100% of the normal range of motion of apatient and more preferably 70% to 100% of the normal range of motion ofa patient.

In some cases, certain of the deflection rods of a dynamic stabilizationassembly can have a different stiffness or rigidity or flexibility thanother of the deflection rods. Thus, in the same assembly, a firstdeflection rod can have a first flexibility or stiffness or rigidity,and a second deflection rod can have a second different flexibility orstiffness or rigidity depending on the needs of the patient. Particularembodiments of a dynamic stabilization assembly may utilize deflectionrods having different deflection properties for each level and/or sideof the dynamic stabilization assembly. In other words, one portion of adynamic stabilization assembly may offer more resistance to movementthan the other portion based on the design and selection of differentstiffness characteristics, if that configuration benefits the patient.

FIGS. 2A through 2G illustrate the design and operation of a firstembodiment of a deflection rod according to an embodiment of the presentinvention. FIG. 2A shows an exploded view of deflection rod 200.Deflection rod 200 includes retainer 202, deflectable post 204, sleeve206, shield 208, collar 210, screw 212 and ball 214. Deflection rod 200connects to vertical rod 216 at a ball joint which includes ball 214,pocket 218 and cap 220. Shield 208 and collar 210 are securely attachedto each other (or formed in one piece) and make up the mount 207. Athreaded aperture 211 passes obliquely through collar 210. Threadedaperture 211 is configured to receive a screw 212. Sleeve 206 is made ofa compliant material which permits movement of deflectable post 204relative to shield 208. Deflectable post 204 may thus pivot in anydirection about the center of ball-shaped retainer 202 as shown byarrows 230. The sleeve 206 controls and limits the deflection of thedeflectable post 204. The deflectable post 204 can also rotate about thelongitudinal axis of the post and the bone anchor as shown by arrow 232.

Referring now to FIG. 2B, which shows a perspective view of a fullyassembled deflection rod 200. When assembled, deflectable post 204 ispositioned within sleeve 206; sleeve 206 is positioned within shield208. Ball 214 is connected to the proximal end of deflectable post 204to provide a component of a ball joint for connecting deflection rod 200to a vertical rod 216. Ball 214 may be formed in one piece withdeflectable post 204 or may be securely attached to deflectable post 204using a joint, for example, a threaded joint, welded joint, adhesivejoint. Retainer 202 is attached to the distal end of deflectable post204 to prevent deflectable post 204 from being pulled out of sleeve 206.

As shown in FIG. 2A, the retainer 202 may be a ball-shaped retainer 202.Retainer 202 may be formed in one piece with deflectable post 204 or maybe securely attached to deflectable post 204. The retainer 202 may beattached by laser welding, soldering or other bonding technology. Forexample, retainer 202 in the form of a ball, disk, plate or other shapemay be laser welded to the distal end of deflectable post 204.Alternatively, retainer 202 may mechanically engage the deflectable post204 using, for example, threads. For example, a lock ring, toothedlocking washer, cotter pin or other mechanical device can be used tosecure deflectable post 204 within shield 208.

The ball 214 of deflection rod 200 is received in a pocket of verticalrod 216. Cap 220 secures ball 214 within the pocket of vertical rod 216creating a ball joint 222 which allows vertical rod 216 to rotate 360degrees around the axis of deflectable post 204 (as shown by arrow 234)and also tilt away from the plane perpendicular to the axis ofdeflectable post 204 (as shown by arrow 236). Thus, the vertical rod 216is allowed to rotate and/or have tilting and/or swiveling movementsabout a center which corresponds with the center of ball 214 of balljoint 222. Ball 214 can also be displaced relative to shield 208 bydeflection of deflectable post 204 (as shown by arrows 230).

FIG. 2C shows a sectional view of a fully assembled deflection rod 200along the axis indicated by line C-C of FIG. 2B. As shown in FIG. 2C,sleeve 206 occupies the space between deflectable post 204 and shield208 and is compressed by deflection of deflectable post 204 towardsshield 208 in any direction. In some embodiments, sleeve 206 may beformed separately from deflection rod 200. For example, deflectable post204 and sleeve 206 may be press fit into shield 208. Alternatively oradditionally, a biocompatible adhesive may be used to bond the sleeve206 to the shield 208 and/or deflectable post 204. Alternatively, sleeve206 may be formed in place by positioning the deflectable post 204within the shield 208 and then filling the space between the deflectablepost 204 and the shield 208 with liquid polymer (polymer reagents) andallowing the polymer to solidify (polymerize).

FIG. 2C, also illustrates the internal detail of ball joint 222 whichconnects vertical rod 216 and deflectable post 204 of deflection rod200. Vertical rod 216 includes disk-shaped pocket 218 at one end. Theproximal end of deflectable post 204 is passed through aperture 219 indisk-shaped pocket 218 of the vertical rod 216. The diameter ofdeflectable post 204 is smaller than the diameter of aperture 219. Oncethe proximal end of deflectable post 204 is passed through the aperture219, ball 214 is attached to deflectable post 204 using threading,fusing, gluing, press fit and/or laser welding techniques, for example.The diameter of the aperture 219 is less than the diameter of ball 214to prevent ball 214 from passing back through aperture 219. Once ball214 is positioned within the disk-shaped pocket 218 of vertical rod 216,cap 220 is threaded, fused, glued, press fit and/or laser welded, forexample, into pocket 218 thereby securing ball 214 within disk shapedpocket 218. FIG. 2C also shows an optional ridge 209 on the interior ofshield 208 for retaining sleeve 206.

FIG. 2D shows a sectional view of a fully assembled deflection rod 200along the axis indicated by line D-D of FIG. 2B. As shown in FIG. 2D,sleeve 206 occupies the space between deflectable post 204 and shield208 and is compressed by deflection of deflectable post 204 towardsshield 208 in any direction. Sleeve 206 resists deflection ofdeflectable post 204 outwardly from a position that is collinear withthe longitudinal axis of sleeve 206. The dimensions and material ofsleeve 206 may be adjusted to generate the desired deflection/loadcharacteristics for the deflection rod.

FIGS. 2E and 2F illustrate deflection of deflectable post 204. Applyinga force to ball-joint 222 causes deflection of deflectable post 204relative to mount 207 including shield 208 (and any bone anchor to whichit may be mounted). Initially deflectable post 204 pivots about a pivotpoint 203 indicated by an X. In this embodiment pivot point 203 islocated at the center of ball-shaped retainer 202. In other embodimentshowever, pivot point may positioned at a different location. As shown inFIG. 2E, deflection of deflectable post 204 initially compresses thematerial of sleeve 206 between deflectable post 204 and shield 208. Theforce required to deflect deflectable post 204 depends upon thedimensions of deflectable post 204, sleeve 206 and shield 208 as well asthe attributes of the material of sleeve 206.

By changing the dimensions of deflectable post 204, sleeve 206 andshield 208, the deflection characteristics of deflection rod 200 can bechanged. The stiffness of components of the deflection rod can be, forexample, increased by increasing the diameter of the post and/or bydecreasing the diameter of the inner surface of the shield anddeflection guide. Additionally, increasing the diameter of the post willincrease the stiffness of the deflection rod while decreasing thediameter of the post will decrease the stiffness of the deflection rod.Alternatively and/or additionally changing the materials which comprisethe components of the deflection rod can also affect the stiffness andrange of motion of the deflection rod. For example, making sleeve 206out of stiffer and/or harder material reduces deflection of deflectablepost 204.

The stiffness of the deflection rod may thus be varied or customizedaccording to the needs of a patient. The deflection characteristics ofthe deflection rod can be configured to approach the natural dynamicmotion of the spine, while giving dynamic support to the spine in thatregion. It is contemplated, for example, that the deflection rod can bemade in stiffness that can replicate a 70% range of motion andflexibility of the natural intact spine, a 50% range of motion andflexibility of the natural intact spine and a 30% range of motion andflexibility of the natural intact spine. In some cases, a kit isprovided to a doctor having a set of deflection rods with differentforce/deflection characteristics from which the doctor may select thedeflection rods most suitable for a particular patient. In other cases,the surgeon may select deflection rods prior to the procedure based uponpre-operative assessment.

Sleeve 206 is preferably made of a compliant biocompatible polymer.Sleeve 206 may, for example, be made from a polycarbonate urethane (PCU)such as Bionate®. If the sleeve is comprised of Bionate®, apolycarbonate urethane or other hydrophilic polymer, the sleeve can alsoact as a fluid-lubricated bearing for rotation of the deflectable post204 relative to the longitudinal axis of the deflectable post 204 (seearrow 232 of FIG. 2B). In a preferred embodiment, the sleeve is made ofPCU, is 2 mm thick when uncompressed and may be compressed to about 1 mmin thickness by deflection of the post.

The sleeve may also include polymer regions having different properties.For example, the sleeve can include concentric rings of one or morepolymers with each ring having a different hardness of stiffness ordurometer. For example, each successive ring from the center outward canhave a higher hardness or stiffness or durometer so that as the post isdeflected outwardly from a position that is collinear with thelongitudinal axis of the sleeve provides increased resistance to furtherdeflection. The sleeve may also be designed to provide different forcedeflection characteristics in different directions. The deflectable postcould also be designed so that less resistance occurs with increaseddeflection of the post.

As shown in FIG. 2F, after further deflection, deflectable post 204comes into contact with limit surface 228 of shield 208. Limit surface228 is oriented such that when deflectable post 204 makes contact withlimit surface 228, the contact is distributed over an area to reducestress on deflectable post 204 and limit surface 228. As depicted, limitsurface 228 is configured such that as the deflectable post 204 deflectsinto contact with limit surface 228, limit surface 228 is aligned/flatrelative to deflectable post 204 in order to present a larger surface toabsorb any load and also to reduce stress on deflectable post 204 andlimit surface damage. Additional deflection may cause elasticdeformation of deflectable post 204. Because deflectable post 204 isrelatively stiff, the force required to deflect deflectable post 204increases significantly after contact of deflectable post 204 withshield 208. In a preferred embodiment, deflectable post 204 may deflectfrom 0.5 mm to 2 mm in any direction before making contact with limitsurface 228. More preferably, deflectable post 204 may deflectapproximately 1 mm before making contact with limit surface 228.

Thus, as load or force is first applied to the deflection rod by thespine, the deflection of the deflection rod responds about linearly tothe increase in the load during the phase when deflection of deflectablepost 204 causes compression of sleeve 206 as shown in FIG. 2E. Afterabout 1 mm of deflection, when deflectable post 204 contacts limitsurface 228 (as shown in FIG. 2F) the deflection rod becomes stiffer.Thereafter, a greater amount of load or force needs to be placed on thedeflection rod in order to obtain the same incremental amount ofdeflection that was realized prior to this point because furtherdeflection requires bending of deflectable post 204. Accordingly, thedeflection rod provides a range of motion where the load supportedincreases about linearly as the deflection increases and then withincreased deflection the load supported increases more rapidly in anon-linear manner in order to provide stabilization. Put another way,the deflection rod becomes stiffer as the deflection/load increases. Ina dynamic stabilization assembly incorporating the deflection rod, theload sharing and deflection is provided by the deflection rod betweenthe deflectable post and the bone screw or the overall bone anchor suchas bone anchor 102 and to a lesser degree or not in the vertical rodsuch as the vertical rod 106 (FIG. 1B).

FIG. 2G is a sectional view illustrating the implantation of deflectionrod 200 in a vertebra 240. As shown in FIG. 2G, bone anchor 102 isoriented such that is passes through pedicle 242 into vertebral body244. Note that the length of bone anchor 102 is selected based upon theanatomy of the patient. Thus shorter bone anchors are used in smallervertebrae and longer bone anchors are used in larger vertebrae. As shownin FIG. 2G, bone anchor 102 has shallower threads 250 adjacent housing130. Threads 250 engage the harder cortical bone 246 on the surface ofthe vertebra 240. Bone anchor 102 has deeper threads 252 towards thedistal end of bone anchor 102. Threads 252 engage the softer cancellousbone 248 within the vertebral body 244.

As shown in FIG. 2G, deflection rod 200 is mounted within bone anchor102 such that pivot point 203 is positioned below the surface ofvertebra 240. Deflectable post 204 pivots about this pivot point 203positioned within vertebra 240. This is advantageous in that it placespivot point 203 of deflectable post 204 closer to the vertebral body 244and thus closer to the natural instantaneous center of rotation of thespine. Placing pivot point 203 closer to the vertebral body 244 promotesnatural motion and reduces non physiological forces on the bones andstrain on the system. Placing the pivot point 203 closer to thevertebral body 244 also helps isolate bone anchor 102 from the relativemotion between vertebra 240 and the vertical rod 216 which connects onevertebra to another vertebra. Pivot point 203 is preferably at or belowthe surface of the vertebra and more preferably pivot point 203 iswithin the cancellous bone 248 of the vertebrae 240. Even morepreferably, the pivot point 203 is positioned with the pedicle 242 ofthe vertebra 240. In some cases, pivot point 203 may be positionedwithin vertebral body 244.

Alternative Deflection Rods/Loading Rods

FIGS. 3A-3H illustrate a first alternative deflection rod 300. FIG. 3Ashows an exploded view of alternative deflection rod 300. Deflection rod300 includes ball-shaped retainer 302, deflectable post 304, sleeve 306,shield 308, collar 310, and mount 314. In this embodiment, retainer 302is a spherical structure formed in one piece with deflectable post 304.Mount 314, in this embodiment, is the proximal end of deflectable post304 suitable for connecting to a vertical rod. A ball may be used inplace of mount 314 as previously described. In this embodiment, mount314 is formed in one piece with deflectable post 304 and retainer 302.In alternative embodiments, deflectable post 304 may be formedseparately from and securely attached to one or more of mount 314 andretainer 302 by laser welding, soldering or other bonding technology.Alternatively, deflectable post 304 may be formed separately andmechanically engage one or more of mount 314 and retainer 302 using, forexample, threads. For example, a lock ring, toothed locking washer,cotter pin or other mechanical device can be used to secure deflectablepost 304 to one or more of mount 314 and retainer 302.

Sleeve 306 is made of a compliant material which permits movement ofdeflectable post 304 relative to shield 308. The sleeve 306 effectivelycontrols and limits the deflection of the deflectable post 304. Sleeve306 is preferably made of a compliant biocompatible polymer such as PCUby way of example only. The properties of the material and dimensions ofthe sleeve 306 are selected to achieve the desired force/deflectioncharacteristics for deflectable post 304. In a preferred embodiment, thesleeve is made of PCU (Bionate® 80A) and is 2 mm thick when uncompressedand may be compressed to about 1 mm in thickness by deflection of thepost. Sleeve 306 may also be shaped to modify the compliance of sleeve306, for example by providing flutes 307. Sleeve 306 fits inside shield308 surrounding deflectable post 304.

Deflection rod 300 is configured to be mounted in a bone anchor 320,which comprises a bone screw 322 connected to a housing 330. Housing 330has a cavity 332 oriented along the axis of bone anchor 320 at theproximal end and configured to receive deflection rod 300. Housing 330also has an outer surface 334 adapted for mounting a component e.g. anoffset connector. Housing 330 may in some embodiments be cylindrical aspreviously described. As shown in FIG. 3A, outer surface 334 of housing330 is provided with splines/flutes 336. Splines/flutes 336 may beengaged by a driver that mates with splines/flutes 336 for implantingbone anchor 320.

Referring now to FIG. 3B, which shows a perspective view of a deflectionrod 300 assembled with a bone anchor 320. When assembled, deflectablepost 304 is positioned within sleeve 306 of FIG. 3A; sleeve 306 ispositioned within shield 308 of FIG. 3A. Deflectable post 304, sleeve306 and shield 308 are then placed in the cavity 332 of FIG. 3A of boneanchor 320. Threaded collar 310 is then secured in the threaded proximalend of cavity 332. Threaded collar 310 has two sockets 311 for receivingthe pins of a pin wrench to allow threaded collar 310 to be tightened tothreads 338 of housing 330. Threaded collar 310 is laser welded tohousing 330 after installation to further secure the components.Threaded collar 310 secures deflectable post 304, sleeve 306 and shield308 within cavity 332 of bone anchor 320.

FIG. 3C shows a sectional view of a deflection rod 300 assembled with abone anchor 320 along the axis indicated by line C-C of FIG. 3B. Asshown in FIG. 3C, sleeve 306 occupies the space between deflectable post304 and shield 308 and is compressed by deflection of deflectable post304 towards shield 308 in any direction. Retainer 302 fits into ahemispherical pocket 339 in the bottom of cavity 332 of housing 330.Shield 308 includes a flange 309 which secures ball-shaped retainer 302within hemispherical pocket 339 while allowing rotation of ball-shapedretainer 302. Collar 310 secures both shield 308 and sleeve 306 withinhousing 330. If sleeve 306 is comprised of Bionate®, a polycarbonateurethane or other hydrophilic polymer, sleeve 306 can act as a fluidlubricated bearing and allow the post to also rotate about thelongitudinal axis of the post and the bone anchor. Other materials andconfigurations can also allow the post to rotate about the longitudinalaxis of the post and the bone anchor.

FIG. 3D illustrates the deflection of deflectable post 304. Applying aforce to mount 314 causes deflection of deflectable post 304 ofdeflection rod 300. Initially deflectable post 304 pivots about a pivotpoint 303 indicated by an X. Deflectable post 304 may pivot about pivotpoint 303 in any direction. Concurrently or alternatively, deflectablepost 304 can rotate about the long axis of deflectable post 304 (whichalso passes through pivot point 303). In this embodiment, pivot point303 is located at the center of ball-shaped retainer 302. As shown inFIG. 3D, deflection of deflectable post 304 initially compresses thematerial of sleeve 306. The force required to deflect deflectable post304 depends upon the dimensions of deflectable post 304, sleeve 306 andshield 308 as well as the attributes of the material of sleeve 306.

After further deflection, deflectable post 304 comes into contact withlimit surface 313 of collar 310. Limit surface 313 is oriented such thatwhen deflectable post 304 makes contact with limit surface 313, thecontact is distributed over an area to reduce stress on deflectable post304. After deflectable post 304 comes into contact with limit surface313, further deflection requires deformation (bending) of deflectablepost 304. In a preferred embodiment, deflectable post 304 is a titaniumpost 5 mm in diameter. Deflectable post 304 is relatively stiff, and theforce required to deflect deflectable post 304 therefore increasessignificantly after contact of deflectable post 304 with collar 310. Ina preferred embodiment, deflectable post 304 may deflect from 0.5 mm to2 mm in any direction before making contact with limit surface 313. Morepreferably, deflectable post 304 may deflect approximately 1 mm beforemaking contact with limit surface 313.

The inner diameter of the collar 310 may be different in differentcollars so that the distance between limit surface 313 and deflectablepost 304 is different in different deflection rods. This allows for themanufacture of deflection rods having a larger or smaller range ofdeflection before contact between the post and the limit surface. Inthis way, deflection rods may be manufactured having different ranges ofmotion. Moreover, the distance between limit surface 313 and deflectablepost 304 need not be the same in all directions such that the range ofmotion of the deflection rod is different in different directions.

Referring to FIG. 3D, as load or force is first applied to thedeflection rod 300 by the spine, the deflection of deflectable post 304responds about linearly to the increase in the load during the phasewhen deflection of deflectable post 304 causes compression of sleeve306. After about 1 mm of deflection, deflectable post 304 contacts limitsurface 313 and the deflection rod becomes substantially stiffer. Agreater amount of load or force needs to be placed on the deflection rodin order to obtain the same amount of incremental deflection that wasrealized prior to this point because further deflection requires bendingof deflectable post 304. The amount of deflection caused by the loadapplied is a non-linear function, in this embodiment. The deflection rodprovides a range of motion where the load supported increases aboutlinearly as the deflection increases and then with increased deflectionthe load supported increases more rapidly (upon contact of the post withthe limit surface). Alternatively, if desired, this embodiment could bedesigned such that the rate of change of the amount of deflection couldbe a linear function for a larger range of motion by; for example,increasing the distance between limit surface 313 and deflectable post304.

FIG. 3E is a sectional view illustrating the implantation of adeflection rod 300 in a vertebra 240. As shown in FIG. 3E, bone anchor320 is oriented such that is passes through pedicle 242 into vertebralbody 244. Note that the length of bone anchor 320 is selected based uponthe anatomy of the patient. Thus shorter bone anchors are used insmaller vertebrae and longer bone anchors are used in larger vertebrae.As shown in FIG. 3E, housing 330 of bone anchor 320 is mounted entirelyabove the surface of vertebra 240. Pivot point 303 of deflection rod 300is positioned within housing 330 such that pivot point 303 is, in thisembodiment, positioned close to but outside of vertebra 240.

In an alternative embodiment, as shown in FIG. 3F, deflectable post 304pivots about a pivot point 353 positioned within vertebra 240. This isadvantageous in that it places pivot point 353 of deflectable post 304closer to the vertebral body 244 and thus the natural instantaneouscenter of rotation of the spine. Placing the pivot point 353 closer tothe vertebral body 244 promotes natural motion and reducesnon-physiological forces on the bones and strain on the dynamicstabilization assembly. In particular, placing the pivot point 353closer to the vertebral body 244 helps isolate bone anchor 320 from therelative motion between vertebra 240 and a vertical rod of the dynamicstabilization assembly which connects one level of the spine to theadjacent level. Pivot point 353 is preferably at or below the surface ofthe vertebra 240. More preferably, the pivot point 353 is positionedwith the pedicle 242 of the vertebra 240. In some cases, pivot point 353may be positioned within vertebral body 244.

FIG. 3G shows a lateral view of a dynamic stabilization assemblyutilizing deflection rod 300. As shown in FIG. 3G, deflection rod 300 isinstalled in bone anchor 320. Bone anchor 320 is implanted in onevertebra 370 (see e.g. FIG. 3E). A polyaxial screw 350 is implanted in asecond vertebra 372. A vertical rod 360 is secured at one end to mount314 of deflection rod 300. Mount 314 in this embodiment passes throughan aperture in vertical rod 360. The proximal end of mount 314 isthreaded so that vertical rod 360 may be secured to mount 314 with athreaded nut 362. In this embodiment, as shown in FIG. 3G, the verticalrod 360 is secured rigidly to deflectable post 304. The rigid connectionprovides a relatively stiff assembly. However, where greater range ofmotion is desired, deflectable post 304 may be provided with a ball endand vertical rod 360 may be connected to deflectable post 304 by a balljoint as previously described with respect to FIGS. 1A-1B.

Vertical rod 360 is mounted at the other end to the polyaxial head 352of polyaxial screw 350. This screw may be a standard polyaxial screw,for example, a 5.5 mm polyaxial screw available in the marketplace. Thisscrew may, alternatively, be a bone anchor with a polyaxial head e.g.the polyaxial head previously described with respect to FIG. 1C. In apreferred embodiment, vertical rod 360 is a titanium rod 5.5 mm indiameter as used in rigid spinal implants. The vertical rod 360 issecured to polyaxial head 352 using a threaded fitting, set screw 354,for example. The vertical rod 360 thereby supports the vertebrae whiledeflection rod 300 provides for load sharing and allows relative motionof vertebra 370 relative to vertebra 372. Thus, the dynamicstabilization assembly provides dynamic stabilization of the spine. Thedynamic stabilization assembly may be expanded to two or more levelsusing an offset connector mounted to the housing 330 of bone anchor 320.It is to be understood that an offset connector can include a flutedring to assist in engaging the housing 330 (see e.g. shape of openwrench 380 in FIG. 3H). Thus, a modular system is provided whichprovides for the creation of a multi-level dynamic stabilizationassembly.

FIG. 3H illustrates an open wrench 380 for driving bone anchor 320 intoposition. Bone anchor 320 of FIG. 3H has a housing 330. A deflection rod300 is installed in housing 330 and secured in place by threaded collar310 (FIGS. 3A and 3B). Threaded collar 310 engages threads interior tohousing 330. Collar 310 has two apertures 311 which may be engaged by apin wrench to tighten collar 310 to housing 330. Collar 310 may also bewelded to housing 330 to further secure deflection rod 300 with housing330. In this embodiment deflection rod 300 is designed to bepreassembled with bone anchor 320 prior to implantation.

As shown in FIG. 3H, the exterior surface 334 of housing 330 is providedwith surface features in the form of a plurality of splines 336. Splines336 are oriented parallel to the longitudinal axis of bone anchor 320and project from housing 330 at regular intervals. Open wrench 380 has ahead 382 designed to engage the exterior surface 334 of housing 330.With such a tool, the housing 330 can be engaged and rotated about thelongitudinal axis of the bone anchor 320 in order to drive the boneanchor into the bone. Open wrench 380 may be provided with a torquelimiting or torque measuring component to facilitate installation ofbone anchor 320. In alternative embodiments a socket may be used toengage housing 330 in place of an open wrench.

FIG. 3I shows a plan view of bone anchor 320 and deflection rod 300observed from the deflection rod end of the assembly. As shown in FIG.3I there are 16 splines 336 evenly spaced around the exterior surface334 of housing 330. The diameter of collar 310 is the same or smaller asthe minimum diameter of housing 330 in the region of the splines 336 toallow engagement of the splines 336 by a complementary tool or connectorwithout interference from collar 310. In other embodiments there may bea greater or lesser number of splines.

FIG. 3I shows a sectional view of a socket wrench 384 suitable forengaging housing 330. Socket wrench 384 has a plurality of splines 386complementary to splines 336 of housing 330. Socket wrench 384 maytherefore be slipped over deflection rod 300 and housing 330 andpositioned as shown in FIG. 3I. When in position, socket wrench 384 maybe used to rotate housing 330 to install bone anchor 320 in a bone (orremove the bone anchor from the bone). Socket wrench 384 should becomplementary in interior profile to the exterior profile 334 of housing330. Socket wrench 384 need not have as many splines 386 as housing 330has splines 336 so long as splines 386 are correctly positioned toengage some or all of the splines 336 of housing 330. An open wrench orother driver may be designed with the same engagement surface to engagesome or all of the splines 336 of housing 330.

Likewise, connectors that engage the housing of a bone anchor may alsobe readily adapted to engage splines 336 of housing 330. By way ofexample, FIG. 3J shows connector 170 of FIG. 1D adapted to engagesplines 336. Connector 170 mounts externally of the housing 330 of abone anchor 320. The components of connector 170 shown in FIG. 3Jinclude locking set screw 176, clamp ring 180 and saddle 182. As shownin FIG. 3J, clamp ring 180 has, on the inside diameter, a plurality ofsplines 396 complementary to splines 336 of housing 330. Clamp ring 180may therefore be slipped over deflection rod 300 and housing 330 andpositioned as shown in FIG. 3J after implantation of bone anchor 320 ina vertebra. Splines 396 engage splines 336 of housing 330. Clamp ring180 is prevented by splines 396 and 336 from free rotation aroundhousing 330. This is advantageous in that increases the stability of thedynamic stabilization assembly by preventing the clamp ring 180 fromslipping around housing 330 under load. When clamp ring 180 ispositioned at the desired angle relative to bone anchor 320, set screw176 may be tightened onto a vertical rod (not shown) to clamp thevertical rod to the saddle 182 and also tighten clamp ring 180 againstthe exterior surface 334 of housing 330. Thus connector 180 may be usedto securely attach a vertical rod to the housing 330 of bone anchor 320.

Clamp ring 180 (and thus connector 170) may be installed in any of 16positions around housing 330 (22.5 degrees separation betweenpositions). If smaller granularity of positioning is required, a largernumber of splines 336 may be used. Clamp ring 180 should becomplementary in interior profile to the exterior surface 334 of housing330. Clamp ring 180 need not have as many splines 396 as housing 330 hassplines 336 so long as the splines 396 are correctly positioned toengage some or all of the splines 336 of housing 330. A clamp ring 180as shown in FIG. 1D without any splines may still be used to engagehousing 330.

Other connectors may be similarly adapted to engage the splines 336 ofhousing 330 of bone anchor 320. Likewise, the other bone anchorsdiscussed herein may be provided with splines on the exterior of thehousing to facilitate installation and enhance the mounting ofconnectors. In alternative embodiments, different surface features maybe utilized on the surface of a housing for engagement by a tool orconnector. For example, a housing may be made polygonal in exteriorsection and have 8, 3, 12, 16 or more sides. A tool or connector for usewith such a housing would have a complementary interior profile designedto engage the 8, 3, 12, 16 or more sides. Alternatively, a housing maybe provided with a plurality of apertures at regular intervals. A toolor connector for use with such a housing may be provided with a one ormore of pins designed to engage the apertures in a plurality ofpositions in the manner of a pin wrench. Conversely the housing may beprovided with one or more protruding pins and the tool or connector witha plurality of complementary apertures. Alternatively, one or both ofthe housing and connector may be provided with shallow surface featuressuch as dots, dimples, ridges or the like designed to increase thefrictional engagement of the housing and connector. In the latter case,the features of the housing and connector need not necessarily becomplementary to one another and the connector and housing may be freeto engage one another at any angular position.

One feature of embodiments of the present invention is load sharingprovided by the deflection rod. The deflection rod provides stiffnessand support where needed to support the loads exerted on the spineduring normal spine motion thereby recovering improved spine functionwithout sacrificing all motion. The deflection rod also isolates theanchor system components from forces exerted by the dynamicstabilization assembly thereby reducing stress on the bone anchors andthe bone to which they are attached. In particular embodiments, thedeflection rods of the present invention are oriented coaxial with thelongitudinal axis of the bone anchor to which they are attached or inwhich they are incorporated. Moreover, by selecting the appropriatestiffness of the deflection rod or loading rod to match the physiologyof the patient and the loads that the patient places on the spine, abetter outcome is realized for the patient.

In order to utilize deflection rods of the present invention toconstruct a dynamic stabilization assembly, the deflection rod iscoupled with a vertical rod. The deflection rod may be coupled to thevertical rods in a fixed, pivoting or flexible manner depending on therequirements of the dynamic stabilization assembly. One mechanism forcoupling a deflection rod to a vertical rod is the ball-joint 222illustrated for example in FIGS. 2A-2C and FIGS. 2E-2G. As shown in FIG.2B, the vertical rod 216 is coupled to the deflectable post 204 by theball-joint 222 in a manner that allows the vertical rod 216 to rotateabout the long axis of the deflectable post 204 and also pivot relativeto the deflectable post 204. These two degrees of freedom are presentboth during implantation and also in the completed dynamic stabilizationassembly. By comparing FIGS. 2C, 2E and 2F, it can be seen that theangle between the vertical rod 216 and deflectable post 204 changes asdeflectable post 204 is deflected. This change in angle is accommodatedby rotation of ball 214 in ball joint 222.

A second mechanism for coupling a deflection rod to a vertical rod isthe threaded mount 314 of deflection rod 300 illustrated in FIGS. 3A-3H.As shown in FIG. 3G, the vertical rod 360 is secured to threaded mount314 by a nut 362. The vertical rod 360 can be rotated around mount 314before nut 362 is tightened but, thereafter, vertical rod 360 is rigidlysecured to deflectable post 304. After completion of the dynamicstabilization assembly, vertical rod can still rotate around the longaxis of bone anchor 320 because deflectable post 304 may rotate relativeto the long axis of bone anchor 320. However, the angle between verticalrod 360 and deflectable post 304 is fixed. Thus, any angle changebetween vertical rod 360 and deflectable post 304 resulting frommovement of the vertebra must be accommodated by deformation (bending)of vertical rod 360 and deflectable post 304. Vertical rod 360 anddeflectable post 304 are relatively stiff and thus, the dynamicstabilization assembly is stiff as compared to a dynamic stabilizationassembly which may accommodate the angle change without bending of thevertical rod and deflectable post using e.g. a ball-joint. Thus, themechanism by which the vertical rod is coupled to a deflection rodaffects the ease by which the dynamic stabilization system may beassembled and also the stiffness of the dynamic stabilization assembly.

Alternative Bone Anchor and Compound Spinal Rod

FIGS. 4A-4C illustrate a bone anchor and a compound spinal rod whichcooperate to closely approximate the natural kinematics of the spinediscussed above. FIGS. 4A-4C illustrate a preferred embodiment of a boneanchor 400. FIGS. 5A-5D illustrate a preferred embodiment of a compoundspinal rod 500. FIGS. 6A-6C illustrate the combined kinematics of boneanchor 400 combine with compound spinal rod 500 in a dynamicstabilization prosthesis 600.

FIG. 4A shows an exploded view of bone anchor 400. FIG. 4B shows aperspective view of bone anchor 400, as assembled. FIG. 4C shows asectional view of bone anchor 400. Referring first to FIG. 4A, boneanchor 400 includes, in this embodiment, three components: bone screw420, deflectable post 440, and cap 410. Bone screw 420 comprises athreaded shaft 422 with a housing 430 at one end. Housing 430 may insome embodiments be cylindrical as previously described and is in someembodiments provided with splines/flutes. Housing 430 is preferablyformed in one piece with threaded shaft 422. Housing 430 has a cavity432 oriented along the axis of threaded shaft 422. Cavity 422 is open atthe proximal end of housing 430 and is configured to receive deflectablepost 440.

In a preferred embodiment, deflectable post 440 is a titanium post 5 mmin diameter. Deflectable post 440 has a retainer 442 at one end. At theother end of deflectable post 440 is a mount 444. Retainer 442 is aball-shaped or spherical structure in order to form part of a linkageconnecting deflectable post 440 to bone screw 420. Mount 444 is a lowprofile mount configured to connect deflectable post 440 to a verticalrod component (not shown, but see, e.g. FIGS. 5A-5C). Mount 444comprises a threaded cylinder 446 to which the vertical rod componentmay be secured. Mount 444 in some embodiments also comprises a polygonalsection 445 to prevent rotation of a component relative to mount 444.

Mount 444 includes a male hex extension 448 which may be engaged by atool to hold stationary mount 444 during attachment to a vertical rod.At the proximal end of male hex extension is a nipple 449 for securingmale hex extension 448 into a tool. Hex extension 448 is breakawaycomponent. Between hex extension 448 and threaded cylinder 446 is agroove 447. Groove 447 reduces the diameter of deflectable post 440 suchthat hex extension 448 breaks away from threaded cylinder 446 when adesired level of torque is reached during attachment of a vertical rod.The breakaway torque is determined by the diameter of remaining materialand the material properties. In a preferred embodiment the breakawaytorque is approximately 30 foot pounds. Thus, hex extension 448 breaksaway during implantation and is removed. Nipple 449 is engaged by thetool in order to remove hex extension 448. Deflectable post 440 is alsoprovided with flats 443 immediately adjacent mount 444. Flats 417 allowdeflectable post 440 to be engaged by a tool after hex extension 448 hasbeen removed.

Referring again to FIG. 4A, a cap 410 is designed to perform multiplefunctions including securing retainer 442 in cavity 432 of bone anchor420. Cap 410 has a central aperture 412 for receiving deflectable post440. In the embodiment of FIG. 4A, cap 410 has surface features 414, forexample splines or flutes, adapted for engagement by an implantationtool or mounting a component, e.g. an offset connector. Surface features414 may be, for example, engaged by a driver that mates with surfacefeatures 414 for implanting bone anchor 400 in a bone. As shown in FIG.4A, cap 410 comprises a cylindrical shield section 418 connected to acollar section 416. Shield section 418 is designed to mate with cavity432 of housing 430. Shield section 418 is threaded adjacent collarsection 416 in order to engage threads at the proximal end of cavity 432of housing 430. The distal end of shield section 418 comprises a flange419 for securing retainer 442 within cavity 432 of housing 430.

Bone anchor 400 is assembled prior to implantation in a patient. FIG. 4Bshows a perspective view of bone anchor 400 as assembled. Whenassembled, deflectable post 440 is positioned through cap 410. Cap 410is then secured to the threaded end of cavity 432 (see FIGS. 4A and 4C)of housing 430 of bone anchor 420. Cap 410 has surface features 414 forengagement by a wrench to allow cap 410 to be tightened to housing 430.For example, cap 410 may be hexagonal or octagonal in shape or may havesplines and/or flutes and/or other registration elements. Cap 410 mayalternatively or additionally be laser welded to housing 430 afterinstallation. Cap 410 secures deflectable post 440 within cavity 432 ofbone anchor 420. Deflectable post 440 extends out of housing 430 and cap410 such that mount 444 is accessible for connection to a vertical rod.Bone anchor 400 is implanted in a bone in the configuration shown inFIG. 4B and prior to attachment of a vertical rod or other spinal rod. Aspecial tool may be used to engage the surface features 414 of cap 410during implantation of bone anchor 400 into a bone (See, e.g. FIGS.13A-13D).

FIG. 4C shows a sectional view of a bone anchor 400. Retainer 442 fitsinto a hemispherical pocket 439 in the bottom of cavity 432 of housing430. The bottom edge of cap 410 includes the curved flange 419 whichsecures ball-shaped retainer 442 within hemispherical pocket 439 whileallowing ball-shaped retainer 442 to pivot and rotate. Accordingly, inthis embodiment, a ball-joint is formed. FIG. 4C also illustratesdeflection of deflectable post 440—dashed lines. Applying a force tomount 444 causes deflection of deflectable post 440 of bone anchor 400.Deflectable post 440 pivots about a pivot point 403 indicated by an X.Deflectable post 440 may pivot about pivot point 403 in any direction,as shown by arrow 450. Concurrently or alternatively, deflectable post440 can rotate, as shown by arrow 452, about the long axis ofdeflectable post 440 (which also passes through pivot point 403). Inthis embodiment, pivot point 403 is located at the center of ball-shapedretainer 442. In a preferred embodiment, deflectable post 440 maydeflect from 0.5 mm to 2 mm in any direction before making contact withlimit surface 413. More preferably, deflectable post 440 may deflectapproximately 1 mm before making contact with limit surface 413. After afixed amount of deflection, deflectable post 440 comes into contact withlimit surface 413 of cap 410. Limit surface 413 is oriented such thatwhen deflectable post 440 makes contact with limit surface 413, thecontact is distributed over an area to reduce stress on deflectable post440. In this embodiment, the deflectable post 440 contacts the entiresloping side of the conically-shaped limit surface 413. In anotherembodiment, the deflectable post may only contact a limit ring that islocated distally from the flange 419 of cap 410. After deflectable post440 comes into contact with limit surface 413, further deflectionrequires deformation (bending) of deflectable post 440.

FIGS. 5A-5C show exploded, perspective, and sectional views of analternative embodiment of a compound spinal/vertical rod. Referringfirst to FIG. 5A, compound rod 500 includes a coupling 510 joined by apin 502 to a rod 530. As shown in FIG. 5A, coupling 510 has a mount 512at one end and a clevis 518 at the other end. Mount 512 is configured tobe secured to a bone anchor. Mount 512 includes a bore 514 therethroughsized to receive the mount of a bone anchor (see e.g. mount 444 of FIGS.4A and 4B). Bore 514 is in some embodiments configured to mate with themount of a bone anchor to preclude rotation—for example by beingpolygonal in section. However, in alternative embodiments bore 514 iscircular in section. Coupling 510 is adapted to be secured to a boneanchor using, for example a threaded nut. Coupling 510 is, in someembodiments, provided with a recess 516 to reduce the profile of a nutabove coupling 510.

Coupling 510 is connected to clevis 518 by offset or dogleg connector526. The dogleg connector 526, in addition to the other components,provides for enhanced motion of a spinal prosthesis so that theprosthesis can model the natural kinetics of the spine (See, e.g. FIG.5C). Clevis 518 has two arms 520 separated by a slot 522. Each arm 520has an aperture 521 for receiving pin 502. Slot 522 is size to receive adisc 532 formed at one end of rod 530. Disc 532 also has an aperture 534for receiving pin 502. Thus rod 530 may rotate relative to coupling 510about the axis of rotation of pin 502. The axis of rotation of pin 502,in this embodiment, is substantially perpendicular to the axis of bore514 except that the pin axis is offset from the bore axis.

FIG. 5B shows compound rod 500 as assembled. Compound rod 500 isassembled prior to implantation in a patient. Disc 532 is placed in slot522 between arms 520. Aperture 534 is aligned with apertures 521. Pin502 is then inserted between arms 520, across slot 522 and throughaperture 534 thereby securing disc 532 within slot 522. Pin 502 can besecured mechanically or bonded to clevis 518 by e.g. laser welding.

FIG. 5C shows a sectional view of compound rod 500 as assembled andmounted to the mount 444 of bone anchor 400 of FIGS. 4A-4C. As shown inFIG. 5C mount 512 of coupling 510 is secured to mount 444 of deflectablepost 440 by a nut 511. Coupling 510 is also joined by pin 502 to rod530. Mount 512 of coupling 510 has a bore 514 for receiving the mount444 of a bone anchor 400. After assembly, rod 530 is free to pivotrelative to coupling 510 around the axis of pin 502 as shown by arrow538. Further, it is noted that the pivot pin 502 and the pivot axis islocated to the side of the housing 430 and substantially perpendicularto and offset from the longitudinal axis of the threaded shaft 422 ofthe bone anchor 400. Further, the pivot pin 502 is located below thelevel where the compound rod 500 is connected to the deflectable post440 of the bone anchor 400. The mount 444 and pin 502 are approximatelyequidistant from pin 502 of compound rod 500 and pivot about pivot point403. However, dogleg connector 526 changes the position of pin 502relative to mount 444. The shape of the dogleg connector 526 controlsthe angle between the mount 444 and pivot point 403 relative to pin 502and thus can be designed to modulate the direction of movement of pivotpoint 403. The length of the dogleg connector controls the distancebetween the pin 502 and pivot point 403 and thus can be designed tomodulate the amount of movement of pivot point 403 for a given amount ofdeflection of coupling 510. The kinematics of pivot point 403 enabled bypin 502 and dogleg connector 526 permits a spinal prosthesis to moreclosely approximate the natural kinematics of the spine by couplingrotation or coupling 510 with translation of pivot point 403 as shown byarrow 540.

FIG. 5D shows a lateral view of a spinal stabilization prosthesis 540utilizing the bone anchor 400 of FIGS. 4A-4C in combination with thecompound rod 500 of FIGS. 5A-5C. As shown in FIG. 5D, a spinalprosthesis 540 can be used to stabilize three vertebrae 542 a, 542 b,and 542 c. Bone anchor 400 is implanted in pedicle 544 a of vertebra 542a. Conventional pedicle screws 550 b, 550 c are implanted in pedicles544 b, 544 c of vertebrae 542 b, and 542 c. Coupling 512 of compound rod500 is secured to mount 444 of deflectable post 440 of bone anchor 400by nut 546. Rod 530 is positioned in the heads 552 b, 552 c or pediclescrews 550 b, 550 c and secured with set screws 554 b, 554 c. The spinalstabilization prosthesis 540 secures vertebra 542 b in fixedrelationship to vertebra 542 c and is suitable for posteriorstabilization of fusion between vertebra 542 b and 544 c. Rod 530 isalso in fixed relationship with vertebra 542 b and 544 c. Likewise pin502 is in fixed relationship to rod 530. The spinal stabilizationprosthesis permits controlled movement of vertebra 542 a relative to 542b while providing load sharing. Controlled movement of vertebra 542 arelative to vertebra 542 b is enabled by pivoting of coupling 510relative to rod 530 (see FIG. 5 C) in combination with pivoting androtation of deflectable post 440 relative to bone anchor 420 (see FIG. 4C).

In an alternative embodiment of a spinal prosthesis, a shorter rod 530is used and compound rod 500 spans two vertebra from a bone anchor 400to a single conventional pedicle screw implanted in an adjacentvertebra. Typically, identical or similar stabilization structures areimplanted on each side of the spinal column. Furthermore, althoughcompound rod 500 has been shown in combination with bone anchor 400 ofFIGS. 4A-4C, compound rod 500 can, in other embodiments, be utilizedwith any other of the bone anchors having deflectable posts describedherein.

The bone anchor 400 has a low profile. As a result, compound rod 500 ismounted closer to the surface of the vertebrae. Moreover, the shape ofcompound rod 500 places rod 530 several millimeters closer to thesurface of the vertebrae. Often, the level of the vertical rod is usedto guide the depth of placement of the pedicle screw in the adjacentvertebrae. Although the vertical rods can be bent by the surgeon tocompensate for any height offset this process is technically difficult.Thus, the surgeons often prefer to arrange the various pedicle screwswith the mounting points in alignment the vertical rod without bending.The low profile of bone anchor 400 and compound rod 500 allowconventional pedicle screw used in conjunction therewith to be mountedwith all of threaded shaft implanted in the vertebra and head abuttingthe surface of the vertebra. This is the preferred location as itreduces stress on the vertebra and conventional pedicle screw byincreasing the contact area between pedicle screw and vertebra andreducing the moment arm.

Thus, one of the advantages of bone anchor 400 of FIGS. 4A-4C is the lowprofile which causes lees trauma to tissue and a better position of thesystem and better alignment with a pedicles screw fully implanted in anadjacent vertebra. The compound rod 500 of FIGS. 5A-5C enhances the lowprofile configuration which causes lees trauma to tissue and a betterposition of the system and better alignment with a pedicles screw fullyimplanted in an adjacent vertebra. In a preferred embodiment the top ofthe cap of bone anchor 400 is approximately 10 mm above the surface ofthe vertebra when implanted and the proximal side (furthest fromvertebrae) of rod 530 is approximately 11 mm above the surface of thevertebra. Also in the preferred embodiment the coupling 510 is no morethan about 15 mm from the surface of the vertebrae when implanted.

FIGS. 6A-6C illustrate the kinetics of a spinal prosthesis 600 having aconventional pedicle screw 610 joined by a compound rod 500 to a boneanchor 400. As shown in FIG. 6A, coupling 510 of compound rod 500 issecured in fixed relationship to deflectable post 440. Coupling 510 anddeflectable post 440 thus move as one unit. Likewise rod 530 is securedin fixed relationship to pedicle screw 610. Rod, 530 and pedicle screw610 thus move as one unit. Bone anchor 400 can move in a controlledmanner with respect to pedicle screw 610 by pivoting of coupling 510(and deflectable post 440) relative to rod 530 (and pedicle screw 610)in combination with pivoting and rotation of bone screw 420 relative todeflectable post 440 (and coupling 510). In a preferred embodiment, thegap between the distal surface of rod 530 and a line joining the pediclesurface on adjacent vertebra is less than about 10 mm. More preferablythe gap between the distal surface of rod 530 and a line joining thepedicle surface on adjacent vertebra is less than about 10 mm.

FIG. 6A shows the movement of bone screw 420 relative to pedicle screw610 assuming no motion within bone anchor 400. As shown in FIG. 6A, bonescrew 420 can pivot relative to pedicle screw 610 as shown by arrow 650.Bone screw 420 can move over a wide range of movement, e.g. ±90 degreesfrom parallel with pedicle screw 610. However, the axis of the rotationis the axis of pin 502 which is offset from the axis of bone screw 420by a distance controlled by the length of coupling 510. The length andshape of coupling 510 causes pivoting of coupling 510 to producepivoting of bone screw 420 and also net translation of bone screw 420relative to pedicle screw 610 as shown by arrow 652.

FIG. 6B shows the movement of bone screw 420 relative to pedicle screw610 assuming no motion within compound rod 500. As shown in FIG. 6B,bone screw 420 can pivot ±10 degrees from the axis of deflectable post440 (and pedicle screw 610) as shown by arrow 654. Bone screw pivotsabout The bone screw pivots about pivot point 403 within deflectablepost 440. Bone screw 420 can also rotate around its long axis relativeto deflectable post 440 as shown by arrow 656.

FIG. 6C is a simplified graph illustrate the combined kinetics enabledby bone anchor 400 when combined with compound rod 500. As shown in FIG.6C, bone anchor 400 and compound rod 500, when combined, allow complexkinetics that more closely approximate the natural kinetics of the spinethan either component alone. For example, as shown in FIG. 6C, spinalstabilization prosthesis 600 supports coupling of spinalflexion/extension (arrow 658) with dorsal-ventral translation (arrow660). Moreover, spinal stabilization prosthesis 600 also supportsmovement of bone screw 420 about a natural center of rotation 662.Although not shown, spinal stabilization prosthesis 600 also supportscoupling of other movement axes, for example, the coupling of lateralbending with axial rotation.

FIG. 7A illustrates a preferred embodiment of a vertical rod 710 for usewith deflection rod 300. As shown in FIG. 7A, vertical rod 710 comprisesa rod 711 which is preferably a 5.5 mm diameter titanium rod. Verticalrod 710 has a pocket 712 at one end sized to receive a ball 720. Ball720 is preferably a cobalt chrome ball. Ball 720 has a polygonalaperture 722 designed to closely engage the polygonal section 702 ofmount 314. Ball 720 is inserted into pocket 712 and secured into placewith threaded cap 730. Pocket 712 is threaded to receive cap 730. Ball720 is placed in pocket 712 and then cap 730 is screwed into thethreaded portion of pocket 712. Cap 730 is preferably titanium and maybe laser welded or otherwise secured to vertical rod 710 after assembly.The components of vertical rod 710—titanium rod 711, titanium cap 730and cobalt chrome ball 720 are assembled prior to use.

FIGS. 7B and 7C shows a sectional view through vertical rod 710. FIG. 7Bshows ball 720 positioned within pocket 712 of rod 711. As shown in FIG.7B cap 730 and pocket 712 capture ball 730 such that it cannot beremoved from vertical rod 710. Ball 730 can, however, rotate 360 degreesaround the axis of aperture 722 as shown by arrow 750. This allowsvertical rod 710 to rotate 360 degrees around the long axis of thedeflection rod or bone anchor to which ball 730 is mounted. Ball 730 canalso tilt from the position shown in FIG. 7B as shown in FIG. 7C byarrows 752. In a preferred embodiment ball 730 can tilt 7 degrees in anydirection therefore allowing vertical rod 710 to tilt 7 degrees fromperpendicular relative to the deflection rod or bone anchor to whichball 730 is mounted. Note that the mount 314 and a nut to secure thevertical rod 710 to mount 314 are designed so not as to interfere withthe range of motion either in rotation or tilting (See FIG. 3A).

Vertical rod 710 may be used with a standard bone anchor, a deflectionrod and bone anchor (for example bone anchor 320 and deflection rod 300of FIG. 3A), or a polyaxial screw. Likewise, the assembly of deflectionrod 300 and bone anchor 320 of FIG. 3A may be utilized with vertical rod710, but may also be utilized in conjunction with a vertical rod nothaving a ball joint.

Alternative Bone Anchor and Compound Spinal Rod

FIGS. 8A-8D illustrate another alternative bone anchor 800. FIG. 8Ashows an exploded view of bone anchor 800. FIG. 8B shows a perspectiveview of bone anchor 800, as assembled. FIG. 8C shows a sectional view ofbone anchor 800. FIG. 8D illustrates deflection of the deflectable postof bone anchor 800.

Referring first to FIG. 8A, bone anchor 800 includes, in thisembodiment, four components: bone screw 820, deflectable post 840,centering rod 860, and cap 810. Bone screw 820 comprises a threadedshaft 822 with a housing 830 at one end. Housing 830 may in someembodiments be cylindrical as previously described and is in someembodiments provided with splines/flutes. Housing 830 is preferablyformed in one piece with threaded shaft 822. Housing 830 has a cavity832 oriented along the axis of threaded shaft 822. Cavity 832 is open atthe proximal end of housing 830 and is configured to receive deflectablepost 840.

Centering rod 860 is, in a preferred embodiment, a cylindrical rod madeof a superelastic metal—for example nitinol. The proximal end 862 ofcentering rod 860 is sized and configured to be received withindeflectable post 840. The distal end 864 of centering rod 860 is sizedand configured to be received within bone screw 820. In a preferredembodiment both centering rod 860 is cylindrical in shape. However, inalternative embodiments, the proximal end 862 and distal end of rod 860may have other than a circular section, for example, square, oval,rectangular, or other polygonal. Note that the distal end 864 andproximal end 862 of centering rod 860 can have the same, or different,sectional shapes. The center section 866 of centering rod 860 isdesigned to bend in response to deflection of deflectable post 840relative to bone screw 820 and exert a restorative centering force upondeflectable post 840. The restorative force tends to align thelongitudinal axis of the deflectable post 840 with the longitudinal axisof the bone screw 820. The force increases as the angle between thedeflectable post 840 and bone screw 820 increases. The diameter andshape of center section 866 of centering rod 860 can bedesigned/selected to achieve a desired restorative force for a givendeflection. Center section 866 can be cylindrical but may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal. The force/deflection response can accordingly beisotropic or anisotropic depending upon the shape of center section 866.In one embodiment, centering rod 860 is a superelastic nitinol wirehaving a diameter between 0.060 and 0.080 inches. In an exemplaryembodiment centering rod is a superelastic nitinol wire having adiameter of 0.063 inches.

A hemispherical pocket 839 (shown by dashed lines) is formed in thebottom of cavity 832 of housing 830. A bore 834 extends distally fromthe bottom of hemispherical pocket 839 along the longitudinal axis ofbone screw 820. Bore 834 is sized and configured to receive the distalend 864 of centering rod 860. Bore 834 is chamfered where it meetshemispherical pocket 839 to allow for bending of centering rod 860. In apreferred embodiment both centering rod 860 and bore 834 are cylindricalin shape such that the distal end 864 of centering rod 860 may rotateabout its longitudinal axis within bore 834. However, in alternativeembodiments, the distal end 864 of rod 860 and bore 834 may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal.

In a preferred embodiment, deflectable post 840 is a titanium post 5 mmin diameter. Deflectable post 840 is alternatively made of cobaltchrome. Deflectable post 840 has a retainer 842 at one end. At the otherend of deflectable post 840 is a mount 844. Retainer 842 is aball-shaped or spherical structure in order to form part of a linkageconnecting deflectable post 840 to bone screw 820. Mount 844 is a lowprofile mount configured to connect deflectable post 840 to a verticalrod component (not shown, but see, e.g. FIGS. 5A-5C). Mount 844comprises a threaded cylinder 846 to which the vertical rod componentmay be secured. Mount 844 in some embodiments also comprises a polygonalsection 845 to prevent rotation of a component relative to mount 844.

A bore 870 (show by dashed lines) extends proximally from the bottom ofball-shaped retainer 842 along the longitudinal axis of deflectable post840. Bore 870 includes a proximal bore 872 sized and configured toreceive the proximal end 862 of centering rod 860. Bore 870 has a largerdistal bore 876. Distal bore 876 is sized to allow bending of centeringrod 860 and deflection of deflectable post 840. In some embodiments,distal bore 876 is sized such that center section 866 does not contactthe sides of distal bore 876 over the full range of motion ofdeflectable post 840. In alternative embodiments, distal bore 876 issized and shaped such that center section 866 comes into contactprogressively with the sides of distal bore 876 over the range of motionof deflectable post 840 thereby modulating the centering force. Distalbore 876 is chamfered where it intersects the surface of retainer 842.In a preferred embodiment both centering rod 860 and proximal bore 872are cylindrical in shape such that the proximal end 862 of centering rod860 may rotate about its longitudinal axis within proximal bore 872.However, in alternative embodiments, the proximal end 862 of rod 860 andproximal bore 872 may have other than a circular section, for example,square, oval, rectangular, or other polygonal. Note that the distal end864 and proximal end 862 of centering rod 860 can have the same, ordifferent, sectional shapes.

On the proximal end of deflectable post 840 is a mount 844 forconnecting deflectable post 840 to a vertical rod or other component.Mount 844 includes a male hex extension 848 which may be engaged by atool to hold stationary mount 844 during attachment to a vertical rod.At the proximal end of male hex extension is a nipple 849 for securingmale hex extension 848 into a tool. Hex extension 848 is breakawaycomponent. Between hex extension 848 and threaded cylinder 846 is agroove 847. Groove 847 reduces the diameter of deflectable post 840 suchthat hex extension 848 breaks away from threaded cylinder 846 when adesired level of torque is reached during attachment of a vertical rod.The breakaway torque is determined by the diameter of remaining materialand the material properties. In a preferred embodiment the breakawaytorque is approximately 30 foot pounds. Thus, hex extension 848 breaksaway during implantation and is removed. Nipple 849 is engaged by thetool in order to remove hex extension 848. Deflectable post 840 is alsoprovided with a pair of flats 843 immediately adjacent mount 844. Flats843 allow deflectable post 840 to be engaged by a tool if necessaryafter hex extension 848 has been removed (for example to disconnect avertical rod during revision of the implant).

Referring again to FIG. 8A, a cap 810 is designed to perform multiplefunctions including securing retainer 842 in cavity 832 of bone screw820. Cap 810 has a central aperture 812 for receiving deflectable post840. In the embodiment of FIG. 8A, cap 810 has surface features 814, forexample splines or flutes, adapted for engagement by an implantationtool or mounting a component, e.g. an offset connector. Surface features814 may be, for example, engaged by a driver that mates with surfacefeatures 814 for implanting bone anchor 800 in a bone. As shown in FIG.8A, cap 810 comprises a cylindrical shield section 818 connected to acollar section 816. Shield section 818 is designed to mate with cavity832 of housing 830. Shield section 818 is threaded adjacent collarsection 816 in order to engage threads at the proximal end of cavity 832of housing 830. The distal end of shield section 818 comprises a curvedflange 819 for securing retainer 842 within cavity 832 of housing 830.

Bone anchor 800 is assembled prior to implantation in a patient. FIG. 8Bshows a perspective view of bone anchor 800 as assembled. Duringassembly, centering rod 860 (not shown) is received in bore 870 and bore834. As bore 870 and bore 834 are closed, it is not necessary to attachcentering rod 860 to either of deflectable post 840 or bone screw 820.However, if desirable centering rod 860 can be attached to either orboth of deflectable post 840 or bone screw 820 by mechanical means (e.g.pins), welding or other fastening mechanism. Retainer 842 (not shown) isreceived in hemispherical pocket 839 (not shown). Deflectable post 840is then positioned through cap 810. Cap 810 is then secured to thethreaded end of cavity 832 (see FIGS. 8A and 8C) of housing 830 of bonescrew 820. Cap 810 has surface features 814 for engagement by a wrenchto allow cap 810 to be tightened to housing 830. For example, cap 810may be hexagonal or octagonal in shape or may have splines and/or flutesand/or other registration elements. Cap 810 may alternatively oradditionally be laser welded to housing 830 after installation. Cap 810secures retainer 842 within cavity 832 of bone screw 820. Deflectablepost 840 extends out of housing 830 and cap 810 such that mount 844 isaccessible for connection to a vertical rod. Bone anchor 800 isimplanted in a bone in the configuration shown in FIG. 8B and prior toattachment of a vertical rod or other spinal rod. A special tool may beused to engage the surface features 814 of cap 810 during implantationof bone anchor 800 into a bone (See, e.g. FIGS. 13A-13D).

FIG. 8C shows a sectional view of a bone anchor 800 after assembly.Retainer 842 fits into a hemispherical pocket 839 in the bottom ofcavity 832 of housing 830. The bottom edge of cap 810 includes thecurved flange 819 which secures ball-shaped retainer 842 withinhemispherical pocket 839 while allowing ball-shaped retainer 842 topivot and rotate. Accordingly, in this embodiment, a ball-joint isformed. Deflectable post 840 pivots about a pivot point 803 indicated byan X. In a preferred embodiment the pivot point 803 is positioned on thecenter of the section of centering rod 860. Deflectable post 840 maypivot about pivot point 803 in any direction, as shown by arrow 850.Concurrently or alternatively, deflectable post 840 can rotate, as shownby arrow 852, about the long axis of deflectable post 840 (which alsopasses through pivot point 803).

As shown in FIG. 8C, distal end 864 of centering rod 860 is receivedwithin bore 834 of bone screw 820. Proximal end 862 of centering rod 860is received within proximal bore 872 of deflectable post 840. Centersection 866 of centering rod 860 is received within distal bore 876 ofbone screw 820. Note that an annular cavity 874 exists around centersection 866 leaving center section free to flex when deflectable post840 pivots about pivot point 803.

FIG. 8D illustrates deflection of deflectable post 840—dashed lines.Applying a force to mount 844 causes deflection of deflectable post 840of bone anchor 800. Deflectable post 840 pivots about pivot point 803located at the center of ball-shaped retainer 842. Proximal end 862 ofcentering rod 860 remains aligned with deflectable post 840 whereasdistal end 864 remains aligned with bone screw 820. Thus center section866 of centering rod 860 bends elastically in response to deflection ofdeflectable post 840. Centering rod 860 thus applies a centering forceupon deflectable post 840 pushing back into alignment with bone screw820. The magnitude of the force increases as the deflection increases.The magnitude of the force can be selected based on the configurationand material of centering rod 860. For example a larger diametercylindrical nitinol rod will provide a larger centering force than asmaller diameter rod for the same amount of deflection. Note that, inthis embodiment, distal bore 876 is sufficiently large that centersection 866 of centering rod 860 does not contact the sides of distalbore 876 over the range of deflection of deflectable post 840 as limitedby contact with limit surface 813 of cap 810.

In a preferred embodiment, deflectable post 840 may deflect from 0.5 mmto 2 mm in any direction before making contact with limit surface 813.More preferably, deflectable post 840 may deflect approximately 1 mmbefore making contact with limit surface 813. After a fixed amount ofdeflection, deflectable post 840 comes into contact with limit surface813 of cap 810. Limit surface 813 is oriented such that when deflectablepost 840 makes contact with limit surface 813, the contact isdistributed over an area to reduce stress on deflectable post 840. Inthis embodiment, the deflectable post 840 contacts the entire slopingside of the conically-shaped limit surface 813. After deflectable post840 comes into contact with limit surface 813, further deflectionrequires deformation (bending) of deflectable post 840. Bending ofdeflectable post 840 requires significantly more force than bending ofcentering rod 860.

FIGS. 9A-9D illustrate another alternative bone anchor 900. FIG. 9Ashows an exploded view of bone anchor 900. FIG. 9B shows a perspectiveview of bone anchor 900, as assembled. FIG. 9C shows a sectional view ofbone anchor 900. FIG. 9D illustrates deflection of the deflectable postof bone anchor 900.

Referring first to FIG. 9A, bone anchor 900 includes, in thisembodiment, four components: bone screw 920, deflectable post 940,centering rod 960, and cap 910. Bone screw 920 comprises a threadedshaft 922 with a housing 930 at one end. Housing 930 is provided withtool engagement features 936 which are adapted to be engaged by a wrench(not shown) to drive threaded shaft 922 into a bone. Housing 930 ispreferably formed in one piece with threaded shaft 922. Housing 930 hasa cavity 932 oriented along the axis of threaded shaft 922. Cavity 932is open at the proximal end of housing 930 and is configured to receivedeflectable post 940.

Centering rod 960 is, in a preferred embodiment, a cylindrical rod madeof a superelastic metal—for example nitinol. The proximal end 962 ofcentering rod 960 is sized and configured to be received withindeflectable post 940. The distal end 964 of centering rod 960 is sizedand configured to be received within bone screw 920. In a preferredembodiment both ends of centering rod 960 are cylindrical in shape.However, in alternative embodiments, the proximal end 962 and distal endof rod 960 may have other than a circular section, for example, square,oval, rectangular, or other polygonal. Note that the distal end 964 andproximal end 962 of centering rod 960 can have the same, or different,sectional shapes. The center section 966 of centering rod 960 isdesigned to bend in response to deflection of deflectable post 940relative to bone screw 920 and exert a restorative centering force upondeflectable post 940. The restorative force tends to align thelongitudinal axis of the deflectable post 940 with the longitudinal axisof the bone screw 920. The force increases as the angle between thedeflectable post 940 and bone screw 920 increases. The diameter andshape of center section 966 of centering rod 960 can bedesigned/selected to achieve a desired restorative force for a givendeflection. Center section 966 can be cylindrical but may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal. The force/deflection response can accordingly beisotropic or anisotropic depending upon the shape of center section 966.In one embodiment, centering rod 960 is a superelastic nitinol wirehaving a diameter between 0.060 and 0.080 inches. In an exemplaryembodiment centering rod is a superelastic nitinol wire having adiameter of 0.063.

In one embodiment, centering rod 960 is 0.063 inch diameter nitinolwire.

A hemispherical pocket 939 (shown by dashed lines) is formed in thebottom of cavity 932 of housing 930. A bore 934 (shown by dashed lines)extends distally from the bottom of hemispherical pocket 939 along thelongitudinal axis of bone screw 920. Bore 934 is sized and configured toreceive the distal end 964 of centering rod 960. Bore 934 is chamferedwhere it meets hemispherical pocket 939 to allow for bending ofcentering rod 960. In a preferred embodiment both centering rod 960 andbore 934 are cylindrical in shape such that the distal end 964 ofcentering rod 960 may rotate about its longitudinal axis within bore934.

Deflectable post 940 is a post 5 mm in diameter. Deflectable post 940can be made, for example, from cobalt chrome or titanium. Deflectablepost 940 has a retainer 942 at the distal end. Retainer 942 is aball-shaped or spherical structure in order to form part of a linkageconnecting deflectable post 940 to bone screw 920. At the proximal endof deflectable post 940 is a mount 944. Mount 944 is a low profile mountconfigured to connect deflectable post 940 to a vertical rod component(not shown, but see, e.g. FIGS. 5A-5C). Mount 944 comprises a threadedsection 946 to which the vertical rod component may be secured. Mount944 has at the proximal end a socket 949 which can be engaged by awrench during the securing of a vertical rod to mount 944.

A bore 970 (show by dashed lines) extends proximally from the bottom ofball-shaped retainer 942 along the longitudinal axis of deflectable post940. Bore 970 includes a proximal bore 972 sized and configured toreceive the proximal end 962 of centering rod 960. Bore 970 has a largerdistal bore 976. Distal bore 976 is sized to allow bending of centeringrod 960 and deflection of deflectable post 940. In some embodiments,distal bore 976 is sized such that center section 966 does not contactthe sides of distal bore 976 over the full range of motion ofdeflectable post 940. In alternative embodiments, distal bore 976 issized and shaped such that center section 966 comes into contactprogressively with the sides of distal bore 976 over the range of motionof deflectable post 940 thereby modulating the centering force. Distalbore 976 is chamfered where it intersects the surface of retainer 942.In a preferred embodiment both centering rod 960 and proximal bore 972are cylindrical in shape such that the proximal end 962 of centering rod960 may rotate about its longitudinal axis within proximal bore 972.

Referring again to FIG. 9A, a cap 910 is designed to perform multiplefunctions including securing retainer 942 in cavity 932 of bone screw920. Cap 910 has a central aperture 912 for receiving deflectable post940. As shown in FIG. 9A, cap 910 comprises a cylindrical shield section918 connected to a collar section 916. Shield section 918 is designed tomate with cavity 932 of housing 930. The distal end of shield section918 comprises a curved flange 919 for securing retainer 942 withincavity 932 of housing 930. Shield section 918 is threaded adjacentcollar section 916 in order to engage threads at the proximal end ofcavity 932 of housing 930. Cap 910 can be provided with surface featuresfor engagement by tool during attachment of cap 910 to housing 930. Forexample, cap 910 may be hexagonal or octagonal in shape or may havesplines, sockets and/or flutes and/or other registration elements.

Bone anchor 900 is assembled prior to implantation in a patient. FIG. 9Bshows a perspective view of bone anchor 900 as assembled. Duringassembly, centering rod 960 (shown by dashed lines) is received in bore970 and bore 934 (see FIGS. 9A and 9C). Because bore 970 and bore 934are closed, it is not necessary to attach centering rod 960 to either ofdeflectable post 940 or bone screw 920. However, if desired, centeringrod 960 can be attached to either or both of deflectable post 940 orbone screw 920 by mechanical means (e.g. pins), welding or otherfastening method/device. Retainer 942 (not shown) is received inhemispherical pocket 939 (not shown). Deflectable post 940 is thenpositioned through aperture 912 of cap 910. Cap 910 is then secured tothe threaded end of cavity 932 (see FIGS. 9A and 9C) of housing 930 ofbone screw 920. Cap 910 may alternatively or additionally be laserwelded to housing 930 after installation. Cap 910 secures retainer 942within cavity 932 of bone screw 920. Deflectable post 940 extends out ofhousing 930 and cap 910 such that mount 944 is accessible for connectionto a vertical rod. Bone anchor 900 is typically implanted in a bone inthe configuration shown in FIG. 9B and prior to attachment of a verticalrod or other spinal rod. A special tool may be used to engage thesurface features 936 of housing 930 during implantation of bone anchor900 into a bone (See, e.g. FIGS. 13A-13D).

FIG. 9C shows a sectional view of a bone anchor 900 after assembly.Retainer 942 fits into a hemispherical pocket 939 in the bottom ofcavity 932 of housing 930. The distal edge of cap 910 includes thecurved flange 919 which secures ball-shaped retainer 942 withinhemispherical pocket 939 while allowing ball-shaped retainer 942 topivot and rotate. Accordingly, in this embodiment, a ball-joint isformed. Deflectable post 940 pivots about a pivot point 903 indicated byan X. In a preferred embodiment the pivot point 903 is positioned on thecenter of the section of centering rod 960 (at least when thelongitudinal axis of the deflectable post 940 and bone screw 920 arealigned). Deflectable post 940 may pivot about pivot point 903 in anydirection, as shown by arrow 950. Concurrently or alternatively,deflectable post 940 can rotate, as shown by arrow 952, about the longaxis of deflectable post 940 (which also passes through pivot point903).

As shown in FIG. 9C, distal end 964 of centering rod 960 is receivedwithin bore 934 of bone screw 920. Proximal end 962 of centering rod 960is received within proximal bore 972 of deflectable post 940. Centersection 966 of centering rod 960 is received within distal bore 976 ofdeflectable post 940. Note that an annular cavity 974 exists aroundcenter section 966 leaving center section 966 free to flex whendeflectable post 940 pivots about pivot point 903.

Where flexible components are incorporated in a spinal device, oneimportant consideration is the possibility of failure of the flexibleelement during the life of the device. One advantage of the presentdesign of bone anchor 900 is that the centering rod 960 is not reliedupon for securing mount 944 to bone screw. 920. Thus, if centering rod960 fails at some point, mount 944 and any spinal components connectedto it remain attached to bone screw 920. Thus it is an advantage of thepresent design of bone anchor 900 that, the failure mode for the“flexible element” of this design is fundamentally safe.

It is also advantageous that, in the present design centering rod 960 isfully enclosed within bore 970 and bore 934. Thus, where centering rod960 is nitinol, the nitinol is not in direct contact with tissues of thebody. Furthermore, even if centering rod 960 fails, no parts ofcentering rod 960 can migrate past ball 942 into the tissues surroundingbone anchor 900. Thus it is an advantage of the present design of boneanchor 900 that the flexible nitinol element is entirely enclosed withinthe device and not exposed to contact with tissues.

FIG. 9D shows a sectional view of bone anchor 900 and illustratesdeflection of deflectable post 940. Applying a force to mount 944 causesdeflection of deflectable post 940 of bone anchor 900. Deflectable post940 pivots about pivot point 903 located at the center of ball-shapedretainer 942. Proximal end 962 of centering rod 960 remains aligned withdeflectable post 940 whereas distal end 964 remains aligned with bonescrew 920. Thus center section 966 of centering rod 960 bendselastically in response to deflection of deflectable post 940. Centeringrod 960 thus applies a centering force upon deflectable post 940 pushingit back into alignment with bone screw 920. The magnitude of the forceincreases as the deflection increases. The magnitude of the force can beselected based on the configuration and material of centering rod 960.For example a larger diameter cylindrical nitinol rod will provide alarger centering force than a smaller diameter rod for the same amountof deflection. In a preferred embodiments centering rod 960 is floating,that is to say that it is not fixed to one or both of deflectable post940 and bone screw 920. Thus, upon deflection of deflectable post 940,centering rod 960 can slide somewhat in one or both of proximal bore 972and bore 934 such that the centering rod 960 is not placed underlongitudinal tension during deflection of deflectable post 940.

In a preferred embodiment, deflectable post 940 may deflect from 0.5 mmto 2 mm in any direction before making contact with limit surface 913.More preferably, deflectable post 940 may deflect approximately 1 mmbefore making contact with limit surface 913. After a fixed amount ofdeflection, deflectable post 940 comes into contact with limit surface913 of cap 910. Limit surface 913 is oriented such that when deflectablepost 940 makes contact with limit surface 913, the contact isdistributed over an area to reduce stress on deflectable post 940. Inthis embodiment, the deflectable post 940 contacts the entire slopingside of the conically-shaped limit surface 913. After deflectable post940 comes into contact with limit surface 913, further deflectionrequires deformation (bending) of deflectable post 940. Bending ofdeflectable post 940 requires significantly more force than bending ofcentering rod 960.

As previously stated, the deflection/force response of a centering rod(and a ball-joint incorporating such a centering rod) can be customizedbased on the choice of design, dimensions and materials. It iscontemplated, for example, that the deflection rod can be made instiffness that can replicate a 70% range of motion and flexibility ofthe natural intact spine, a 50% range of motion and flexibility of thenatural intact spine and a 30% range of motion and flexibility of thenatural intact spine for providing in a kit for a doctor to use. After aselected amount of deflection a deflectable post (see e.g. deflectablepost 940 of FIGS. 9A-9D) will make contact with a limit surface (forexample 1 mm of deflection. Further deflection then requires bending ofthe deflectable post 940. The deflectable post 940 therefore respondsmore stiffly as the load increases. As the deflection increases, thestiffness of the deflectable post 940 to further deflection is increasedsuch that the force required per unit of additional deflection increasesin response to the load placed on the spine and deflection rod.

Initially, as load or force is first applied to the deflectable post bythe spine, the deflection of the deflectable post rod responds aboutlinearly to the increase in the load. After the post makes contact withthe limit surface, the deflection rod responds more stiffly. In thisregion, a greater amount of load or force needs to be placed on thedeflection rod in order to obtain the same amount of deflection that wasrealized prior to this point. Accordingly, the deflectable post of thisexample provides a range of motion where the load supported increasesabout linearly as the deflection increases and then with increaseddeflection the load supported increases more rapidly in a non-linearmanner. The transition from lower stiffness to higher stiffness regiondepends upon the distance between the deflectable post and the limitsurface of the cap. This distance may be customized as previouslydescribed so that the transition occurs after the desired amount ofdeflection, for example after about 1 mm of deflection or after about 2mm of deflection.

Referring again to FIG. 9D, note that, in this embodiment, distal bore976 is sufficiently large that center section 966 of centering rod 960does not contact the sides of distal bore 976 over the range ofdeflection of deflectable post 940 as limited by contact with limitsurface 913 of cap 910. FIG. 9E shows an alternative embodiment in whichthe distal bore 976 e makes contact with centering rod 960 as deflectionincreases. This contact reduces the effective length of flexible section966 thereby increasing the stiffness of centering rod 960. Thus, theshape of distal bore 976 can be utilized to modulate theforce/deflection response of the centering rod and a bone anchor/devicewhich incorporates the centering rod. In the embodiment of FIG. 9E,distal bore 976 e has a progressive trumpet like shape. However inalternative embodiments the diameter of bore 976 e can be changecontinuously or more rapidly in some regions than in others.

FIG. 10A illustrates schematically an implant component 1000 utilizing aself-centering ball-joint of the type illustrated with respect to FIGS.8A-8D and 9A-9D. FIG. 10A shows a partial sectional view exploded viewof implant component 1000. Referring first to FIG. 10A, implantcomponent 1000 includes, first element 1001, second element 1002, andself-centering ball-joint 1003. Referring to FIG. 10A, first element1001 is illustrated by box A. First element 1001 can be, for example,one of a rod, a coupling, a fastener, a mount, a bone anchor, a bonehook, and a bone screw. Second element 1002 is illustrated by Box B.Second element 1002 can be, for example, one of a rod, a coupling, afastener, a mount, a bone anchor, a bone hook, and a bone screw. Firstelement 1001 is connected to second element 1002 by self-centeringball-joint 1003. Self-centering ball-joint 1003 allows first element1001 to pivot relative to second element 1002 as shown by arrow 1050.Self-centering ball-joint 1003 allows first element 1001 to rotaterelative to second element 1002 as shown by arrow 1052. Self-centeringball-joint 1003 constrains the range of pivoting of first element 1001relative to second element. Self-centering ball-joint 1003 also exerts acentering force in response to pivoting and/or rotation of first element1001 relative to second element 1002. Self-centering ball-joint 1003includes a ball-rod 1010, a housing 1020 and a centering rod 1030.

Ball-rod 1010 includes a ball 1012 and a rod 1014. Ball-rod 1010 can bemade, for example, from cobalt chrome or titanium. Ball 1012 is aball-shaped or spherical structure in order to form part of a linkageconnecting ball-rod 1010 to housing 1020. First element 1001 isconnected to rod 1014 at the opposite end from ball 1012. Ball 1012, rod1014 and first element 1001 are in some embodiments made in one piece.In alternative embodiments, ball 1012, rod 1014 and first element 1001are made in two or more pieces and subsequently attached and/or bondedto one another.

A bore 1040 passes along the longitudinal axis of rod 1014 and passesthrough ball 1012. Bore 1040 includes a first bore 1041 communicatingwith a center bore 1042. First bore 1041 is sized and configured toreceive first end 1031 of centering rod 1030. In a preferred embodimentboth centering rod 1030 and first bore 1041 are cylindrical in shapesuch that the first end 1031 of centering rod 1030 may rotate about itslongitudinal axis within first bore 1041. Center bore 1042 is sized andconfigured to create a space 1044 around center section 1034 ofcentering rod 1030. Space 1044 is sized to allow bending of centeringrod 1030 and deflection of ball-rod 1010 relative to housing 1020. Insome embodiments, space 1044 is sized such that center section 1066 doesnot contact the sides of center bore 1042 over the full range of motionof ball-rod 1010. In alternative embodiments, space 1044 is sized andshaped such that center section 1034 comes into contact progressivelywith the sides of center bore 1042 over the range of motion of ball-rod1010 thereby modulating the centering force/deflection response. Centerbore 1042 is chamfered where it intersects the surface of ball 1012.

Housing 1020 forms a socket 1022 in which ball-rod 1010 is received.Socket 1022 includes a partial-spherical pocket 1024. Partial-sphericalpocket 1024 is sized to receive ball 1012 of ball-rod 1010. A channel1026 passes out of partial-spherical pocket 1024 through the surface ofhousing 1020. Channel 1026 is sized to receive rod 1014 of ball-rod1010. Channel 1026 provides a space 1027 around rod 1014 which allowsball-rod 1010 to pivot relative to housing 1020. Channel 1026 alsoprovides a limit surface 1028 which contacts rod 1014 when rod 1014 haspivoted through a pre-selected angle. Channel 1026 thereby permitspivoting of ball-rod 1010 relative to housing 1020 within constraintsdetermined by the positioning of limit surface 1028. Housing 1020 can beformed in one or more pieces.

Housing 1020 also includes a second bore 1029 which extends from thepartial-spherical pocket 1024 opposite channel 1026. Second bore 1029 issized and configured to receive second end 1032 of centering rod 1030.Second bore 1029 is preferably chamfered where it meetspartial-spherical pocket 1024.

Second element 1002 is connected to housing 1020. Housing 1020 andsecond element 1002 are in some embodiments made in one piece. Inalternative embodiments, housing 1020 and second element 1002 are madein two or more pieces and subsequently attached and/or bonded to oneanother.

Centering rod 1030 is, in a preferred embodiment, a cylindrical rod madeof a superelastic metal—for example nitinol. The first end 1031 ofcentering rod 1030 is sized and configured to be received within firstbore 1041 of ball-rod 1010. The second end 1032 of centering rod 1030 issized and configured to be received within second bore 1029 of housing1020. In a preferred embodiment both ends of centering rod 1030 arecylindrical in shape. However, in alternative embodiments, the first end1031 and second end 1032 may have other than a circular section, forexample, square, oval, rectangular, or other polygonal. Note that thefirst end 1031 and second end 1032 of centering rod 1030 can have thesame, or different, sectional shapes.

The center section 1034 of centering rod 1030 is designed to bend inresponse to deflection of ball-rod 1010 relative to housing 1020 andexert a restorative centering force upon ball-rod 1010. The restorativeforce tends to align the longitudinal axis of the ball-rod 1010 with thelongitudinal axis of the housing 1020. The force increases as the anglebetween the ball-rod 1010 and housing 1020 increases. The diameter andshape of center section 1034 of centering rod 1030 can bedesigned/selected to achieve a desired restorative force for a givendeflection. Center section 1034 can be cylindrical but may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal. The force/deflection response can accordingly beisotropic or anisotropic depending upon the shape of center section1034. In one embodiment, centering rod 1030 is 0.063 inch diameternitinol wire. In alternative embodiment, centering rod 1030 is asuperelastic nitinol wire having a diameter between 0.060 and 0.080inches. However centering rod 1030 can be sized and shaped as necessaryto achieve the desired force/deflection response for the system.

Implant component 1000 is assembled prior to implantation in a patient.During assembly, centering rod 1030 (shown by dashed lines) is receivedin bore 1040 and second bore 1029 (see FIGS. 10A and 10C). As shown inFIG. 10C, first end distal end 1064 of centering rod 1030 is receivedwithin bore 1029 of housing 1020. Proximal end 1062 of centering rod1030 is received within proximal bore 1072 of ball-rod 1010. Centersection 1066 of centering rod 1030 is received within distal bore 1076of ball-rod 1010. Note that an annular cavity 1074 exists around centersection 1066 leaving center section 1066 free to flex when ball-rod 1010pivots about pivot point 1005.

Because bore 1040 and second bore 1029 are closed, it is not necessaryto attach centering rod 1030 to either of ball-rod 1010 or housing 1020.However, if desired, centering rod 1030 can be attached to either orboth of ball-rod 1010 1040 and housing 1020 by mechanical means (e.g.pins), welding or other fastening method/device. Ball 1012 is receivedin partial-spherical pocket 1024. Partial-spherical pocket 1024 isshaped to secure ball 1012 within partial-spherical pocket 1024 whileallowing ball 1012 to pivot and rotate. Accordingly, in this embodiment,a ball-joint is formed. Rod 1014 extends through and out of channel 1026such that first element 1001 is external to housing 1020. Ball-rod 1010pivots about a pivot point 1005 indicated by an X. In a preferredembodiment the pivot point 1005 is positioned on the center of thesection of centering rod 1030 (at least when the longitudinal axis ofthe ball-rod 1010 and housing 1020 are aligned). Ball-rod 1010 may pivotabout pivot point 1005 in any direction, as shown by arrow 1050.Concurrently or alternatively, ball-rod 1010 can rotate, as shown byarrow 1052, about the long axis of ball-rod 1010 (which also passesthrough pivot point 1005).

FIG. 10B illustrates deflection of ball-rod 1010. Applying a force tofirst element 1001 causes deflection of ball-rod 1010 of implantcomponent 1000. Ball-rod 1010 pivots about pivot point 1005 located atthe center of ball 1012. First end 1031 of centering rod 1030 remainsaligned with ball-rod 1010 whereas second end 1032 remains aligned withhousing 1020. Thus center section 1034 of centering rod 1030 bendselastically in response to deflection of ball-rod 1010. Centering rod1030 thus applies a centering force upon ball-rod 1010 pushing it backinto alignment with housing 1020. The magnitude of the force increasesas the deflection increases. The magnitude of the force can be selectedbased on the configuration and material of centering rod 1030. Forexample a larger diameter cylindrical nitinol rod will provide a largercentering force than a smaller diameter rod for the same amount ofdeflection. Note that, in this embodiment, center bore 1042 issufficiently large that center section 1034 of centering rod 1030 doesnot contact the sides of center bore 1044 over the range of deflectionof ball-rod 1010 as limited by contact with limit surface 1028 ofhousing 1020. In a preferred embodiment, centering rod 1030 is floating,that is to say that it is not fixed to one or both of ball-rod 1010 andhousing 1020. Thus, upon deflection of ball-rod 1010, centering rod 1030can slide somewhat in one or both of first bore 1041 and second bore1029 such that the centering rod 1030 is not placed under longitudinaltension during deflection of ball-rod 1010.

In a preferred embodiment, ball-rod 1010 may deflect from 0.5 mm to 2 mmin any direction before making contact with limit surface 1028. Morepreferably, ball-rod 1010 may deflect approximately 1 mm before makingcontact with limit surface 1013. After a fixed amount of deflection,ball-rod 1010 comes into contact with limit surface 1028 of housing1020. Limit surface 1028 is oriented such that when ball-rod 1010 makescontact with limit surface 1028, the contact is distributed over an areato reduce stress on ball-rod 1010. In this embodiment, the ball-rod 1010contacts the entire sloping side of the conically-shaped limit surface1028. After ball-rod 1010 comes into contact with limit surface 1013,further deflection requires deformation (bending) of ball-rod 1010.Bending of ball-rod 1010 requires significantly more force than bendingof centering rod 1030.

FIG. 10C illustrates an alternative configuration of an implantcomponent 1000 c. As shown in FIG. 10C, first element 1001, secondelement 1002 and self-centering ball-joint 1003 need not be arranged inline with one another. In implant component 1000 c, second element 1002is offset from the axis of ball-rod 1010.

Centering Rods

As illustrated in FIGS. 8A to 10C, in preferred embodiments of thepresent invention, the centering rod is a cylinder of a superelasticmetal—for example nitinol. However, centering rods can be manufacturedin a range of different configurations and materials depending upon thedesired force deflection characteristics desired for the ball-joint inwhich they are used. For example, in some embodiments, by adjusting theproperties of the centering rod, the deflection characteristics of abone anchor can be configured to approach the natural dynamic motion ofthe spine, while giving dynamic support to the spine in that region. Itis contemplated, for example, that the flexible bone anchor canreplicate a 70% range of motion and flexibility of the natural intactspine, a 50% range of motion and flexibility of the natural intact spineand a 30% range of motion and flexibility of the natural intact spine.In some cases, a kit is provided to a doctor having a set of flexiblebone anchors with different force/deflection characteristics from whichthe doctor may select the flexible bone anchors most suitable for aparticular patient. In other cases, the surgeon may select bone anchorsprior to the procedure based upon pre-operative assessment. Inembodiments centering rod is designed to maintain a deflectable postcoaxial with the bone anchor during implantation of the bone anchorthereby ensuring that a desirable range of motion/load sharing isprovided.

The stiffness of the centering rod may thus be varied or customizedaccording to the needs of a patient or application. Furthermore, onefeature of the present invention is to allow the efficient manufactureof a range of deflectable bone anchors having a range of differentforce-deflection characteristics. This can readily be accomplished bymanufacturing a range of centering rods having differentforce-deflection characteristics and leaving the remainder of thecomponents unchanged. In this way, the range of deflectable bone anchorsis adapted to be manufactured with a minimum number of unique parts.

FIGS. 11A-11F illustrate alternative designs for centering rods whichcan be utilized in any of the self-centering ball-joints describedherein. FIG. 11A shows a first example of an alternative centering rod1100 a for use in a self-centering ball-joint. Centering rod 1100 a hasa first end 1101 a sized and configured to be received within a bore ofa ball-rod (see, e.g. ball-rod 1010 of FIG. 10A). Centering rod 1100 ahas a second end 1102 a sized and configured to be received within abore of housing (see, e.g. housing 1020 of FIG. 10A). In a preferredembodiment both ends of centering rod 1100 a are cylindrical in shape.However, in alternative embodiments, the ends of centering rod 1100 amay have other than a circular section, for example, square, oval,rectangular, or other polygonal to match the bore of the housing andball-rod. Note that the first end 1101 a and second end 1102 a ofcentering rod 1100 a can have the same, or different, sectional shapes.

Referring again to FIG. 11A, centering rod 1100 a has a flexible section1103 a between the first end 1101 a and the second end 1102 a. Flexiblesection 1103 a is designed to bend to allow deflection of the axis offirst end 1101 a relative to the axis of the second end 1102 a. Flexiblesection 1103 a is designed to elastically deform over the designed rangeof motion and exert a restorative centering force to bring the axis offirst end 1101 a back into alignment with the axis of the second end1102 a. The magnitude of the centering force can be selected based onthe design of flexible section 1103 a and the choice of material forflexible section 1103 a. In the embodiment of FIG. 11A, flexible section1103 a is a portion of centering rod 1100 a which has enhancedelasticity or flexibility compared to the rest of centering rod 1100 aby the introduction of a slot or groove 1104 a. Groove 1104 a has aspiral configuration or may have some other configuration adapted toincrease the flexibility of flexible section 1103 a. Flexible section1103 a is in some embodiments formed in one piece with first end 1101 aand second end 1102 a, but may alternatively be formed separately andattached by laser welding, soldering or other bonding technology.Centering rod 1100 a can be formed, for example, from an implantablemetal such as steel, titanium or nitinol.

Groove 1104 a leaves the material of flexible section 1103 a in theshape of a coil spring. By changing the dimensions of the flexiblesection 1103 a and groove 1104 a, the deflection characteristics of thecentering rod 1100 a can be changed. The stiffness of components of thecentering rod can be, for example, increased by increasing the diameterof centering rod 1100 a. Additionally, increasing the amount of materialremoved in groove 1104 a will decrease the stiffness of the centeringrod 1100 a. Alternatively and/or additionally, changing the materialswhich comprise the components of the centering rod 1100 a can alsoaffect the stiffness of the centering rod. For example, making centeringrod 1100 a out of stiffer material reduces deformation of centering rod1100 a for the same amount of load—all other factors being equal.

The centering rod 1100 a may have the same force deflection response ineach direction of deflection of the centering rod (isotropic). Thecentering rod 1100 a may alternatively have different force/deflectionproperties in different directions (anisotropic). For example, thecentering rod 1100 a can have different effective spring constant indifferent directions by adjusting, for example, the thickness of thegroove 1104 a in one region compared to another region.

FIG. 11B shows another example of an alternative centering rod 1100 bfor use in a self-centering ball-joint. Centering rod 1100 b has a firstend 1101 b sized and configured to be received within a bore of aball-rod (see, e.g. ball-rod 1010 of FIG. 10A). Centering rod 1100 b hasa second end 1102 b sized and configured to be received within a bore ofhousing (see, e.g. housing 1020 of FIG. 10A). In a preferred embodimentboth ends of centering rod 1100 b are cylindrical in shape. However, inalternative embodiments, the ends of centering rod 1100 b may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal to match the bore of the housing and ball-rod. Note thatthe first end 1101 b and second end 1102 b of centering rod 1100 b canhave the same, or different, sectional shapes.

Referring again to FIG. 11B, centering rod 1100 b has a flexible section1103 b between the first end 1101 b and the second end 1102 b. Flexiblesection 1103 b is designed to bend to allow deflection of the axis offirst end 1101 b relative to the axis of the second end 1102 b. Flexiblesection 1103 b is designed to elastically deform over the designed rangeof motion and exert a restorative centering force to bring the axis offirst end 1101 b back into alignment with the axis of the second end1102 b. The magnitude of the centering force can be selected based onthe design of flexible section 1103 b and the choice of material forflexible section 1103 b. In the embodiment of FIG. 11B, Flexible section1103 b is cylindrical in shape with an internal cavity 1106 b. Internalcavity 1106 b is made, for example, by drilling from one end ofcentering rod 1100 b. A plurality of apertures 1104 b pierces the wallof flexible section 1103 b into cavity 1106 b. The apertures 1104 b aredesigned to increase the flexibility of flexible section 1103 b ascompared to other regions centering rod 1100 b. In the embodiment shownin FIG. 1100B, apertures 1104 b are shaped to leave material of flexiblesection 1103 b in the form of a multi-level wave spring. Flexiblesection 1103 b is in some embodiments formed in one piece with first end1101 b and second end 1102 b, but may alternatively be formed separatelyand attached by laser welding, soldering or other bonding technology.Centering rod 1100 b can be formed, for example, from an implantablemetal such as steel, titanium or nitinol. In alternative embodiments,the apertures 1104 b and cavity 1106 b are filled with a compliantmaterial, for example a biocompatible polymer such as PEEK or BIONATE™.

FIG. 11C shows another example of an alternative centering rod 1100 cfor use in a self-centering ball-joint. Centering rod 1100 c has a firstend 1101 c sized and configured to be received within a bore of aball-rod (see, e.g. ball-rod 1010 of FIG. 10A). Centering rod 1100 c hasa second end 1102 c sized and configured to be received within a bore ofhousing (see, e.g. housing 1020 of FIG. 10A). In a preferred embodimentboth ends of centering rod 1100 c are cylindrical in shape. However, inalternative embodiments, the ends of centering rod 1100 c may have otherthan a circular section, for example, square, oval, rectangular, orother polygonal to match the bore of the housing and ball-rod. Note thatthe first end 1101 c and second end 1102 c of centering rod 1100 c canhave the same, or different, sectional shapes.

Referring again to FIG. 11C, centering rod 1100 c has a flexible section1103 c between the first end 1101 c and the second end 1102 c. Flexiblesection 1103 c is designed to bend to allow deflection of the axis offirst end 1101 c relative to the axis of the second end 1102 c. Flexiblesection 1103 c is designed to elastically deform over the designed rangeof motion and exert a restorative centering force to bring the axis offirst end 1101 c back into alignment with the axis of the second end1102 c. The magnitude of the centering force can be selected based onthe design of flexible section 1103 c and the choice of material forflexible section 1103 c. In the embodiment of FIG. 11C, flexible section1103 c is a portion of centering rod 1100 c enhanced elasticity orflexibility compared to the rest of centering rod 1100 c because it hasreduced cross-sectional area by removal of material in the region 1104c. A range of centering rods 1100 c can be created having differentcross-sectional areas in flexible section 1103 c and thus havingdiffering spring constants. By keeping first end 1101 c and second end1102 c with a standard diameter while changing only flexible section1103 c, it is possible to create a range of spinal implant componentshaving different force/deflection characteristics while only having tochange one part—the centering rod—of the component. In alternative someembodiments flexible section 1103 c is larger in diameter than first end1101 c and second end 1102 c resulting in an increased spring constantrelative to a cylinder of the same material.

In the embodiment of a centering rod 1100 d shown in FIG. 11D, flexiblesection 1103 c of FIG. 11C is also provided with a sleeve 1104 d. Sleeve1104 d can be formed of a compliant material, for example abiocompatible polymer such as PEEK or BIONATE™. A sleeve 1104 d can beused to modulate the spring constant of flexible section 1103 c, reducewear of metal components, and/or make contact and or fill the spacesurrounding the flexible section 1103 c when installed in a bore of aball-rod. A range of centering rods 1100 d can be created havingdiffering spring constants by changing the diameter of the flexiblesection 1103 c and the sleeve 1104 d. In some embodiments sleeve 1104 dis larger or smaller in diameter than first end 1101 c and second end1102 c.

FIG. 11E shows another example of an alternative centering rod 1100 efor use in a self-centering ball-joint. Centering rod 1100 e has a firstend 1101 e sized and configured to be received within a bore of aball-rod (see, e.g. ball-rod 1010 of FIG. 10A). Centering rod 1100 e hasa second end 1102 e sized and configured to be received within a bore ofhousing (see, e.g. housing 1020 of FIG. 10A). As shown in FIG. 11E, bothends of centering rod 1100 e are hexagonal in section. When received inmatching bores, the hexagonal first end 1101 e and second end 1102 eengage prevent rotation of the first end first end 1101 e and second end1102 e within the bore. Thus, for example, rotation of a ball-rodrelative causes twisting of flexible section 1103 e of the centeringrod. As a consequence, centering rod 1100 e flexibly resists rotation ofe.g. a ball-rod within a socket. the provides a restoring force toreturn the ball-rod Although a hexagonal section is shown in FIG. 11 e,the ends of centering rod 1100 e may have sections designed to engagethe bore in which they are received, for example, square, oval,rectangular, or other polygonal to match the bore of the housing andball-rod. Note that the first end 1101 e and second end 1102 e ofcentering rod 1100 e can have the same, or different, sectional shapes.

FIG. 11F shows another example of an alternative centering rod 1100 ffor use in a self-centering ball-joint. Centering rod 1100 f has a firstend 1101 f sized and configured to be received within a bore of aball-rod (see, e.g. ball-rod 1010 of FIG. 10A). Centering rod 1100 f hasa second end 1102 f sized and configured to be received within a bore ofhousing (see, e.g. housing 1020 of FIG. 10A). As shown in FIG. 11F, bothends of centering rod 1100 f are rectangular in section. When receivedin matching bores, the rectangular first end 1101 f and second end 1102f engage prevent rotation of the first end first end 1101 f and secondend 1102 e within the bore. Thus, for example, rotation of a ball-rodrelative causes twisting of flexible section 1103 f of the centeringrod. As a consequence, centering rod 1100 f flexibly resists rotation ofe.g. a ball-rod within a socket.

Flexible section 1103 f connects first end 1101 f and second end 1102 f.Flexible section 1103 f is designed to permit movement of first end 1101f relative to second end 1102 f. For example, flexible section 1103 fmay by a portion of centering rod 1100 f which has enhanced elasticityor flexibility compared to the rest of centering rod 1100 f. Flexiblesection 1103 f is preferably formed in one piece with second end 1102 fand first end 1101 f or may alternatively be formed separately andattached by laser welding, soldering or other bonding technology.Flexible section 1103 f has a rectangular cross-section which is widerin one direction than the other. Flexible section 1103 f is thus moreflexible bending in a direction parallel to the short axis of therectangular section (see arrow 1140) than in a direction parallel to thelong axis of the rectangular section (see arrow 1142). Thus flexiblesection has an anisotropic force-deflection profile. The centering rod1100 f has different force/deflection properties in different directions(anisotropic). The disparity between the thicknesses of the flexiblesection 1103 f in one direction compared to another can be used tocontrol the anisotropic force/deflection profile of the centering rod1100 f. The anisotropic force/deflection profile of a bone anchorutilizing centering rod 110 f may be useful where it is necessary ordesirable to provider greater or lesser load-sharing and/orstabilization on one axis of spinal motion as compared to another.

Accordingly, the devices of the present invention provide in someembodiments the ability to control stiffness for extension, flexion,lateral bending and axial rotation, and to control stiffness for each ofthese motions independently of the other motions. The characteristics ofthe deflectable post can be changed, for example, by adjusting thediameter of post and/or the properties of the centering rod and/or thedistance between the deflectable post and the limit surface. Thesedeflection characteristics need not be isotropic. A bias can beintroduced in the deflectable post by varying the shape of the bore, theshape of the centering rod and the space between the deflectable postand the limit surface.

For example, by varying the shape of the cap/socket the distance betweenthe deflectable post and the limit surface may also be varied. By makingthe distance shorter, the amount of deflection can be reduced thatoccurs before the increase in stiffness caused by contact with the limitsurface. The cap/socket may be shaped to reduce the gap between the postand the limit surface uniformly or may be shaped to reduce the gapbetween the post and the limit surface more in some directions than inothers (anisotropically).

In embodiments where the deflectable post has anisotropicforce-deflection response, it is important to ensure that thedeflectable post is implanted in the correct orientation. Thedeflectable post is therefore provided with discernable visual orphysical characteristics (e.g. an arrow, color, indentation or otherobservable indicator) which guide the surgeon to the correct orientationof implantation. When correctly installed, a deflectable post withanisotropic force-deflection response may be used to control stiffnessfor extension, flexion, lateral bending and axial rotationindependently. For example, if a deflectable post is more flexible inthe upward direction (relative to the spine after implantation—the headdirection being up), the post can deflect more when the spine is placedin flexion and can deflect less when the spine is placed in extension.In effect, this arrangement is more restrictive with respect to movementof the spine with the spine in extension and less restrictive withrespect to the movement of the spine with the spine in flexion.Conversely, if the deflectable post is more compliant in the downdirection (relative to the spine after implantation—the head directionbeing up), the post can deflect more when the spine is placed inextension and can deflect less when the spine is placed in flexion. Ineffect, this arrangement is more restrictive with respect to movement ofthe spine in flexion and less restrictive with respect to the movementof the spine in extension.

FIG. 12A illustrates a preferred embodiment of a compound spinal rod1200 for use with bone anchor such as bone anchor 800 of FIGS. 8A-8D orbone anchor 900 of FIGS. 9A-9D. As shown in FIG. 12A, compound spinalrod 1200 comprises a rod 1210 and a rod-end 1240.

Rod 1210 is preferably a 5.5 mm diameter titanium rod. Rod 1210 is from50 mm to 150 mm in length. First end 1211 of rod 1210 is design to matewith rod-end 1240. A groove 1214 runs around rod 1210 at first end 1211.Adjacent groove 1214 has a band 1216 around rod 1210 can be knurled orprovided with grooves/ribs to enhance engagement by a set screw 1260.

Rod-end 1240 has a pocket 1242 at one end sized to receive a ball 1220.Ball 1220 is preferably a cobalt chrome ball. Ball 1220 has an aperture1222 designed to engage the mount of a bone anchor. Pocket 1212 isthreaded to receive cap 1230. Cap 1230 is preferably titanium and may belaser welded or otherwise secured to rod-end 1240 after assembly. Ball1220 is inserted into pocket 1212 and secured into place with threadedcap 1230.

Rod end 1240 has a substantially-cylindrical bore 1244 (see dashedlines) at the end opposite pocket 1242. Bore 1244 is configured toreceive first end 1211 of rod 1210. A channel 1246 passes throughrod-end 1240 perpendicular to bore 1244 and intersecting the edge ofbore 1244. Channel 1246 is positioned to correspond with the position ofgroove 1214 when rod 1210 is inserted in bore 1244. Channel 1246 issized to receive a locking pin 1250. When inserted in channel 1246,locking pin 1250 projects into groove 1214 preventing rod-end 1240 frombeing removed from rod 1210. Rod-end 1240 can still rotate around firstend 1211 of rod 1210 and can also slide along rod 1210 with a rangelimited by contact between locking pin 1250 and the sides of groove1214.

Rod-end 1240 also has a threaded bore 1248 which is perpendicular tobore 1244 and intersect bore 1244. Threaded bore 1248 is configured toreceive a set screw 1260. In a preferred embodiment, threaded bore 1248is positioned such that set screw 1260 engages rod 1210 with band 1216.Set screw 1260 is in some embodiments pivotally connected at its distalend to a curved driver element in order to better engage the surface ofrod 1210. Set screw 1260 when tightened is configured to engage firstend 1211 of rod 1210 to lock the position of rod-end 1240 relative torod 1210. To put it another way, set screw 1260 is adapted to engage rod1210 to prevent further rotation and sliding of first end 1211 withinbore 1244.

FIG. 12B shows a perspective view of compound spinal rod 1200 asassembled. As shown in FIG. 12B, ball 1220 has been inserted into pocket1242 of rod-end 1240. Cap 1230 has been installed in pocket 1242securing ball 1220 and forming a ball-joint. Note that ball 1220 canpivot within pocket 1242 as shown by arrows 1261 and can also rotatewithin pocket 1242 as shown by arrow 1262. The aperture 1222 isaccessible from both sides of ball 1220 to allow ball 1220 to be mountedto the mount of a bone anchor.

Referring again to FIG. 12B, first end 1211 of rod 1210 has beeninserted in bore 1244. Locking pin has been inserted through channel1246 to intersect groove 1214. Set screw 1260 has been inserted inthreaded bore 1248. With set screw 1260 loose, rod 1210 can slidelongitudinally within bore 1244 as shown by arrow 1264 within limitsimposed by contact between pin 1250 and the sides of grove 1214 (seeFIG. 12C). In embodiments, rod 1210 has a range of sliding movementbetween 2 and 10 mm. Rod 1210 can also rotate freely within bore 1244 ifgroove 1214 passes around the entire circumference of rod 1210. However,if groove 1214 passes only partway around rod 1210, the rotation of rod1210 within bore 1244 will be constrained by contact between locking pin1250 and the ends of groove 1214. Set screw 1260 when tightened isconfigured to engage first end 1211 of rod 1210 to lock the position ofrod-end 1240 relative to rod 1210.

FIG. 12C shows sectional view of compound spinal rod 1200 as assembled.As shown in FIG. 12C, ball 1220 is held in pocket 1242 of rod-end 1240by cap 1230. Ball 1220 can, however, rotate 360 degrees around the axisof aperture 1222 as shown by arrow 1262. This allows compound spinal rod1200 to rotate 360 degrees around the long axis of the bone anchor towhich ball 1220 is mounted. Ball 1220 can also tilt from the positionshown in FIG. 12B as shown in FIG. 12C by arrows 1260. In a preferredembodiment ball 1220 can tilt 12 degrees in any direction thereforeallowing rod-end 1240 to tilt 12 degrees from perpendicular relative tothe bone anchor to which ball 1220 is mounted. Note that mount and nutused to secure the ball 1220 to a bone anchor are designed so not as tointerfere with the range of motion either in rotation or tilting (See,e.g. FIG. 3A). Locking pin is received in channel 1246 and intersectsgroove 1214 preventing rod-end 1240 from sliding off first end 1211 ofrod 1210. Note that groove 1214 is wider than locking pin 1250 so thatrod end 1240 can slide along rod 1210 until locking pin 1250 contactsthe walls of groove 1214. As shown in FIG. 12C, set screw 1260 ispositioned in threaded bore 1248. The distal end of set screw 1260enters bore 1244 where is can engage rod 1210 to secure the position ofrod-end 1240 relative to rod 1210.

Compound spinal rod 1200 may be used with standard bone anchors,polyaxial screws and/or bone anchors including ball-rods/deflectionrods/deflectable posts as described herein. (See, e.g. bone anchor 800of FIGS. 8A-8D and bone anchor 900 of FIGS. 9A-9D). Likewise, boneanchor 800 of FIGS. 8A-8D and bone anchor 900 of FIGS. 9A-9 d may beutilized with compound spinal rod 1200, but may also be utilized inconjunction with any of the spinal rods described herein and/or spinalrods not having a ball joint. FIG. 12D shows, for example, a lateralview of a spinal stabilization prosthesis 1270 utilizing the bone anchor900 of FIGS. 9A-9D in combination with the compound spinal rod 1200 ofFIGS. 12A-12C. As shown in FIG. 12D, a spinal prosthesis 1270 can beused to stabilize the L4 and L5 vertebrae 1272 a, 1272 b in conjunctionwith a fusion of L5 to the sacrum 1272 c. A fusion of L5 to the sacrumis illustrated schematically by box 1271. Spinal prosthesis 1270 createsa static posterior support of the fusion segment L5-sacrum. Spinalprosthesis 1270 also supports the L4-L5 segment while still permittingsome movement at the L4-L5 segment. Spinal prosthesis 1270 can thus beused in a “topping-off” procedure to support a spinal segment adjacentto a fused segment thereby reducing the likelihood of the increased rateof adjacent segment deterioration sometimes resulting from spinal fusionprocedures.

As shown in FIG. 12D, bone anchor 900 is implanted in pedicle 1274 a ofL4 vertebra 1272 a. Conventional pedicle screw 1280 b is implanted inpedicle 1274 b of L5 vertebrae 1272 b. Conventional pedicle screw 1280 cis implanted in sacrum 1272 c. Ball 1220 of rod-end 1240 is secured tobone anchor 900 by nut 1290. Rod 1210 is positioned in the heads 1282 b,1282 c of pedicle screws 1280 b, and 1280 c. Note that rod 1210 isshaped/bent to support the L4 and L5 vertebrae 1272 a, 1272 b and sacrum1272 c in the desired positions. As shown in FIG. 12D, rod 1210 has abend 1215. Rod 1210 is next secured with set screws to heads 1282 b, and1282 c of pedicle screws 1280 b, 1280 c. The spinal stabilizationprosthesis 1270 thus posteriorly secures L5 vertebra 1272 b in fixedrelationship to sacrum 1272 c and is suitable for posteriorstabilization of fusion between L5 vertebra 1272 b and 1274 c.

Owing to the shape of rod 1210, the act of tightening set screws 1284 b,1284 c to secure rod 1210 to heads 1282 b, 1282 c of pedicle screws 1280b, 1280 c may cause movement of first end 1211 of rod 1210. With somespinal rods this would apply a load to the post of bone anchor 900pushing it away from center. However, rod-end 1240 is free to slidesomewhat relative to rod 1210 and rotate around rod 1210. Thesemovements of rod-end 1240 compensate for any change in position of firstend 1211 of rod 1210 and allow ball 1220 to be positioned directly inline with the longitudinal axis of bone anchor 900. Moreover thecentering rod of bone anchor 900 maintains the deflectable in the centerposition during installation. When rod-end 1240 is properly positioned,set screw 1260 can be tightened as a last step allowing bone anchor 900to be set up in an ideal position for subsequent support of the L4-L5segment. Bone anchor 900—when properly implanted as part of spinalprosthesis 1270 permits controlled movement of L4 vertebra 1272 arelative to L5 1272 b while providing load sharing. Controlled movementof vertebra 1272 a relative to vertebra 1272 b is enabled bypivoting/rotation of ball 1220 within rod end 1240 in combination withpivoting/rotation of the deflectable post relative to the bone anchor.(See FIG. 12 E).

FIG. 12E shows a partial sectional view of a spinal implant prosthesisutilizing the compound spinal rod 1200 of FIGS. 12A-12C in conjunctionwith the bone anchor 900 of FIGS. 9A-9D. FIG. 12E, illustrates spinalimplant prosthesis after securing rod 1210 to pedicle screws 1280 b,1280 c (see FIG. 12D) and after securing rod end 1240 to bone anchor 900with a nut 1290. At this point in implantation the position of first end1211 of rod 1210 is locked in place. Furthermore, the position of bonescrew 920 of bone anchor 900 is also locked in place relative to firstend 1211 of rod 1210. Note however, that deflectable post 1240 of boneanchor 900 may pivot away from the neutral position in which it iscoaxial with bone screw 920 due to movement of first end 1211 of rod1210 during securing of rod 1210. However, rod end 1240 can slide androtate relative to first end 1211 this allows centering rod 960 to pushdeflectable post 940 back into the neutral position where it is coaxialwith bone screw 920 and centered within aperture 912. Once deflectablepost 940 is in the neutral/central position, set screw 1260 is tightenedagainst rod 1210 locking rod-end 1240 in position relative to rod 1210.The movements of rod-end 1240 compensate for any change in position offirst end 1211 of rod 1210 and allow ball 1220 to be positioned directlyin line with the longitudinal axis of bone anchor 900. Moreover thecentering rod 1260 of bone anchor 900 helps maintain the deflectablepost 1240 in the neutral/center position during installation.

In an alternative embodiment of a spinal prosthesis, a shorter rod 1210is used and compound spinal rod 1200 spans two vertebra from bone anchor900 to a single conventional pedicle screw implanted in an adjacentvertebra. Typically, identical or similar stabilization structures areimplanted on each side of the spinal column. Furthermore, althoughcompound spinal rod 1200 has been shown in combination with bone anchor900 of FIGS. 9A-9D, compound spinal rod 1200 can, in other embodiments,be utilized with any other of the bone anchors having deflectable postsdescribed herein.

The bone anchor 900 has a low profile. As a result, compound spinal rod1200 is mounted closer to the surface of the vertebrae. Moreover, theshape of compound spinal rod 1200 places rod 1210 several millimeterscloser to the surface of the vertebrae. Often, the level of the spinalrod is used to guide the depth of placement of the pedicle screw in theadjacent vertebrae. Although the spinal rods can be bent by the surgeonto compensate for any height offset this process is technicallydifficult. Thus, the surgeons often prefer to arrange the variouspedicle screws with the mounting points in alignment the spinal rodwithout bending. The low profile of bone anchor 900 and compound spinalrod 1200 allow conventional pedicle screw used in conjunction therewithto be mounted with all of threaded shaft implanted in the vertebra andhead abutting the surface of the vertebra. This is the preferredlocation as it reduces stress on the vertebra and conventional pediclescrew by increasing the contact area between pedicle screw and vertebraand reducing the moment arm.

Thus, one of the advantages of bone anchor 900 of FIGS. 9A-9D is the lowprofile which causes lees trauma to tissue and a better position of thesystem and better alignment with a pedicles screw fully implanted in anadjacent vertebra. The compound spinal rod 1200 of FIGS. 12A-12Cenhances the low profile configuration which causes lees trauma totissue and a better position of the system and better alignment with apedicles screw fully implanted in an adjacent vertebra. In a preferredembodiment the top of the cap of bone anchor 900 is approximately 10 mmor less above the surface of the vertebra when implanted and theproximal side (furthest from vertebrae) of rod 1210 is approximately 11mm above the surface of the vertebra. Also in the preferred embodimentthe nut 440 is no more than about 15 mm from the surface of thevertebrae when implanted.

Implantation and Assembly Tools

FIGS. 13A-13D and 14A-14F show various steps in the implantation andconnection of a dynamic stabilization assembly utilizing embodiments ofthe bone anchors and spinal rods described herein. Similar methods anddevices can be utilized for the various spinal rods and bone anchorsdescribed herein as modified for the particular tool engagement featuresand fasteners provided thereon.

The implantation and assembly is preferably performed in a minimallyinvasive manner and, thus, tools are provided to facilitate installationand assembly through cannulae. These tools can also be used in openprocedures. One suitable minimally invasive approach to the lumbar spineis the paraspinal intermuscular approach. This approach is described forexample in “The Paraspinal Sacraspinalis-Splitting Approach to theLumbar Spine,” by Leon L. Wiltse et al., The Journal of Bone & JointSurgery, Vol. 50-A, No. 5, July 1968, which is incorporated herein byreference. In general the patient is positioned prone. Incisions aremade posterior to the vertebrae to be stabilized. The dorsal fascia isopened and the paraspinal muscle is split to expose the facet joints andlateral processes of the vertebra. Dynamic bone anchors according toembodiments of the present invention and conventional pedicle screws areplaced in the vertebrae as necessary for the selected assembly. Thescrews are placed lateral to the facet joints and angled in towards thevertebral body. The dynamic rods according to embodiments of the presentinvention are then inserted into position adjacent the dynamic boneanchors according to embodiments of the present invention, screws andconventional pedicle screws. The balls of the dynamic rods according toembodiments of the present invention are then secured to the deflectableposts of the dynamic bone anchors according to embodiments of thepresent invention the other end of the dynamic rod is then connected tothe conventional screws with the desired interpediclular distance. Theimplantation of the dynamic bone anchors and connection of the dynamicrods can be facilitated by the implantation tool (FIGS. 13A-13D) andconnection tool (FIGS. 14A-14F) described below.

FIG. 13A shows a perspective view of an implantation tool 1350 for usein implanting a dynamic bone anchor 1300. Dynamic bone anchor 1300 mayfor example bone anchor 800 of FIGS. 8A-8D of bone anchor 900 of FIGS.9A-9D. Implantation tool 1350 includes an inner shaft 1360 receivedwithin a tubular sleeve 1370. Inner shaft 1360 is free to rotate withinsleeve 1370. Sleeve 1370 may also be slid towards the proximal end ofinner shaft 1360 by pulling on grip 1374. A coil spring 1372 isconnected between the sleeve 1370 and inner shaft 1360 to hold sleeve1370 in its more distal position relative to shaft 1360. The length anddiameter of implantation tool 1350 is selected so as to allow usethrough a cannula in a minimally invasive surgical technique therebyreducing disruption of tissues adjacent the implantation site, reducingpatient recovery and improving surgical outcomes.

Referring again to FIG. 13A, shaft 1360 has at a proximal end a quickrelease mount 1362 to which a handle (not shown) may be attached forturning inner shaft 1360. Suitable handles for attachment to shaft 1360include ratcheting handles, torque sensing handles and torque limitinghandles. In alternative embodiments, a handle may be permanentlyconnected to or integrated with the proximal end of shaft 1362. Innershaft has at a distal end a head 1364. Head 1364 includes means forengaging and securing dynamic bone anchor 1300 during implantation as isdescribed below.

As also shown in FIG. 13A, head 1364 can be received over the proximalportion of dynamic bone anchor 1300 with the ball rod 1306 receivedwithin shaft 1360 (see dashed line). In use, dynamic bone anchor 1300 isinserted into the head 1364 of shaft 1360 with the cap 1310 engaged byhead 1364 and the ball rod 1306 secured within head 1364. Dynamic boneanchor 1300 is thus secured to implantation tool 1350. Dynamic boneanchor 1300 will not be released unless and until the surgeon pulls backon grip 1374. Thus, dynamic bone anchor 1300 and implantation tool canbe inserted as one unit through a cannula to the implantation locationin the spine facilitating the positioning and implantation of dynamicbone anchor 1300.

FIG. 13B shows a detailed sectional view of the head 1364 of theimplantation tool 1350 of FIG. 13A engaged with a dynamic bone anchor1300. As shown in FIG. 13B, head 1364 includes a socket 1365 forreceiving and engaging cap 1310 of dynamic bone anchor 1300. Socket 1365is designed to mate with cap 1310 in order to rotate the threaded shank1320 of dynamic bone anchor 1300. Thus, the interior of socket 1365 maybe hexagonal, octagonal or provided with flutes/splines etc., dependingon the particular configuration of the cap 1310. Socket 1365 should beable to apply sufficient torque to cap 1310 to implant the dynamic boneanchor 1300 in a pedicle.

Referring again to FIG. 13B, head 1364 also includes a bore 1365 forreceiving ball rod 1306 of dynamic bone anchor. As shown in FIG. 13B,ball rod 1306 includes a nipple 1318 at the proximal end. A ball 1352 ispositioned within an aperture 1367 which passes from the exterior ofshaft 1360 intersecting bore 1365 adjacent nipple 1318. Ball 1352 isheld by sleeve 1370 in a position in which ball 1352 protrudes into bore1365 so as to trap nipple 1352 within bore 1365. In a preferredembodiment, there are three such balls, however, only one is shown inthis sectional view. Thus, cap 1310 is received in socket 1365 anddynamic bone anchor 1300 is locked to implantation tool 1350 by theinteraction of nipple 1318 and ball(s) 1352.

FIG. 13C shows a detailed sectional view of the head 1364 of theimplantation tool 1350 of FIG. 13A configured to release a dynamic boneanchor 1300. After implantation of dynamic bone anchor 1300 it isnecessary to remove implantation tool 1350. The first step is to slidesleeve 1370 proximally relative to shaft 1360 as shown by arrow A. Thisis achieved by pulling back on grip 1374 against the force of spring1372 (See FIG. 13A). As sleeve 1360 is pulled proximally, ball(s) 1352enters a portion of sleeve 1360 with a larger internal diameter. Ball(s)1352 can move away from engagement with ball rod 1306 as they pass ramp1365 releasing nipple 1318. At this stage both shaft 1360 and sleeve1370 can be pulled together away from dynamic bone anchor 1300.

FIG. 13D shows a transverse view of the lumbar spine illustrating use ofthe implantation tool 1350 of FIG. 13A to implant dynamic bone anchors1300 in the pedicles 1382 of a lumbar vertebra 1384 according to anembodiment of the invention. As shown in FIG. 13D, implantation tool1350 may be used through a cannula 1380 to implant the dynamic boneanchor in a minimally invasive procedure. The cannula 1380 is introducedto the patient to approach the pedicles posteriorly. The pedicle 1382 ofthe vertebra is 1384 is exposed in the conventional fashion. A hole 1386is then drilled through the pedicle 1382 into the vertebral body 1383 ofthe vertebra. Next a dynamic bone anchor 1300 is selected having ofsuitable length, diameter and force/deflection characteristics isselected for implantation. The cap 1310 of the selected dynamic boneanchor 1300 is inserted into the head 1364 of implantation tool 1350 andsecured in place.

Referring now to the left side of FIG. 13D, dynamic bone anchor 1300 andimplantation tool 1350 are inserted as one assembly through cannula 1380to the implantation site. Then dynamic bone anchor is implanted byturning a handle 1388 attached to the quick release on the proximal endof shaft 1360. The dynamic bone anchor 1300 is driven into hole 1386until the housing is at the surface of the vertebra 1384 (see arrow1390). The torque to drive dynamic bone anchor 1300 is provided byhandle 1388 through shaft 1360 to cap 1310 of dynamic bone anchor 1300.

Referring now to the right side of FIG. 13D, when dynamic bone anchor1300 is correctly positioned in pedicle 1382, the physician pulls backon grip 1374 against the force of spring 1372. Sleeve 1370 movesproximally relative to shaft 1360. Shaft 1360 releases the grip ondynamic bone screw 1300 and the both shaft 1360 and sleeve 1370 moveaway from cannula 1380 and out of the patient (see arrow 1392). Dynamicbone anchor 1300 is now correctly implanted and prepared for attachmentto spinal rod and/or other spinal stabilization assembly components.

FIGS. 14A-14D show views of an attachment tool for securing a spinal rod1400 to a dynamic bone anchor 1300 according to an embodiment of theinvention. FIG. 14A shows a perspective view of an attachment tool 1450for securing a dynamic spinal rod 1400 to a dynamic bone anchor 1300(shown in FIG. 14C) according to an embodiment of the invention. Dynamicspinal rod 1400 may be, for example, the compound spinal rod 500 ofFIGS. 5A-5C, or spinal rod 710 of FIGS. 7A-7C, or compound spinal rod1200 of FIGS. 12A-12E. Dynamic bone anchor 1300 may be, for example,bone anchor 800 of FIGS. 8A-8D or bone anchor 900 of FIGS. 9A-9D.

Referring first to FIG. 14A, attachment tool 1450 includes an innershaft 1460 received within a tubular sleeve 1470. The length anddiameter of attachment tool 1450 is selected so as to allow use througha cannula in a minimally invasive surgical technique thereby reducingdisruption of tissues adjacent the implantation site, reducing patientrecovery time and improving surgical outcomes. Inner shaft 1460 is freeto rotate and slide within sleeve 1470. Inner shaft 1460 has at aproximal end an attached handle 1462. In alternative embodiments shaft1460 may have a fitting to which a handle might be attached, forexample, ratcheting handles, torque sensing handles and torque limitinghandles. Inner shaft has at a distal end a head 1464 for engaging andsecuring the hex extension of a dynamic spinal rod 1400 (see FIG. 14B).

Referring again to FIG. 14A, sleeve 1470 includes a butterfly grip 1474at the proximal end thereof. Sleeve 1470, has at the distal end thereof,means for engaging and securing the female hex socket of a ball of adynamic spinal rod 1400 during connection to a dynamic bone anchor as isdescribed below. In a preferred embodiment head 1464 includes a male hexfitting 1472 with a central aperture 1473. FIG. 14B shows an enlargedview of head 1464 from the distal end of attachment tool 1450. FIG. 14Bshows male hex fitting 1472 with central aperture 1473. Through centralaperture 1473 is visible female hex socket 1465 of head 1464. Protrudinginto female hex socket 1465 are two spring tabs 1467.

FIGS. 14C and 14D show detailed sectional views of the distal endattachment tool 1450 in relation to a dynamic spinal rod 1400 anddynamic bone anchor 1300. Referring first to FIG. 14C, which shows adetailed sectional view of the distal end of the attachment tool 1450 ofFIG. 14A, engaged with a dynamic spinal rod 1400 and a dynamic boneanchor 1300. As shown in FIG. 14C, male hex fitting 1472 of head 1464 ofouter sleeve 1470 fits into the female hex socket of ball 1444. At thesame time a hex extension 1307 of ball rod 1306 is received withinfemale hex socket 1465 of inner shaft 1460. When thus engaged, turninghandle 1462 relative to butterfly grip 1474 (See FIG. 14A) can rotateball rod 1306 relative to ball 1444. Attachment tool 1450 is designed toapply sufficient torque to ball rod 1306 relative to ball 1444 to secureball rod 1306 to ball 1444 and breakaway the hex extension 1307 of ballrod 1306. In a preferred embodiment, attachment tool 1450 should be ableto provide greater than 30 foot pounds of torque.

FIG. 14D shows a detailed sectional view of the distal end of theattachment tool 1450 of FIG. 14A after break away of hex extension 1307of ball rod 1306. As shown in FIG. 14D, when ball 1444 has beentightened onto ball rod 1306, tabs 1467 on central aperture 1473 engageeither side of a nipple 1418 of hex extension 1307 to secure hexextension 1307 within female hex socket 1465. Thus, when hex extension1307 beaks away it can be removed from the patient with connection tool1450 as shown.

FIGS. 14E-14H are lateral views of the lumbar spine illustrating stepsof attaching a dynamic spinal rod 1400 to a dynamic bone anchor 1300utilizing the attachment tool of FIG. 14A according to an embodiment ofthe invention. As shown in FIG. 14E, the dynamic spinal rod 1400 isimplanted after the dynamic bone anchor 1300 and a polyaxial screw 1440have already been implanted. Dynamic spinal rod 1400 is implanted in acranially direction—preferably in a minimally invasive manner untildynamic spinal rod 1400 is positioned adjacent dynamic bone anchor 1300and polyaxial screw 1440. The hex extension 1307 of dynamic bone anchor1300 is then fed through ball 1444 of dynamic spinal rod 1400 as shown.

Next, as shown in FIG. 14F, connection tool 1450 is inserted through acannula 1380 to engage ball 1444 and hex extension 1307. Ball 1444 isthen turned relative to hex extension 1307 until it is fully secured toball rod 1306. When ball 1444 is fully secured to ball rod 1306, furthertorque is applied until hex extension 1307 (not shown) is sheared off.In a preferred embodiment, this requires 30 foot pounds of torque and issufficient to lock ball 1444 to ball rod 1306. Next, as shown in FIG.14G, connection tool 1450 can be removed from cannula 1380. Aspreviously described, hex extension 1307 (not shown) is retained insideattachment tool 1450 for easy removal from the patient. As shown in FIG.14H a conventional tool 1484 is then inserted through cannula 1480 tooperate polyaxial screw 1440 to secure the other end of dynamic spinalrod 1400.

Deflection Rod/Loading Rod Materials

Movement of the deflectable post relative to the bone anchor providesload sharing and dynamic stabilization properties to the dynamicstabilization assembly. As described above, deflection of thedeflectable post deforms the material of the sleeve. In someembodiments, the characteristics of the material of the sleeve incombination with the dimensions of the components of the deflection rodassembly affect the force-deflection curve of the deflection rod. Inother embodiments, the characteristics of the material of the centeringrod in combination with the dimensions of the components of the assemblyaffect the force-deflection curve of the assembly.

The deflectable post, bone anchor, compound rods, centering rods, andspinal rods are preferably made of biocompatible implantable metals. Thedeflectable post can, for example, be made of titanium, titanium alloy,cobalt chrome alloy, a shape memory metal, for example, nitinol (NiTi)or stainless steel. In preferred embodiments, the deflectable post ismade of cobalt chrome alloy. In preferred embodiments, the bone anchorand spinal rods are made of titanium or titanium alloy; however, othermaterials, for example, stainless steel may be used instead of or inaddition to the titanium\titanium alloy components. Furthermore, theball of the dynamic spinal rod is preferably made of cobalt chrome forgood wear characteristics.

The material of the sleeve/compliant member/or-ring (where present) is abiocompatible and implantable polymer having the desired deformationcharacteristics. The material of the sleeve should also be able tomaintain the desired deformation characteristics. Thus the material ofthe sleeve is preferably durable, resistant to oxidation anddimensionally stable under the conditions found in the human body. Thesleeve may, for example be made from a polycarbonate urethane (PCU) suchas Bionate®. If the sleeve is comprised of Bionate®, a polycarbonateurethane or other hydrophilic polymer, the sleeve can also act as afluid-lubricated bearing for rotation of the deflectable post relativeto the longitudinal axis of the deflectable post.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many embodiments were chosenand described in order to best explain the principles of the inventionand its practical application, thereby enabling others skilled in theart to understand the invention for various embodiments and with variousmodifications that are suited to the particular use contemplated.

The particular dynamic stabilization assemblies shown herein areprovided by way of example only. It is an aspect of preferredembodiments of the present invention that a range of components beprovided and that the components may be assembled in differentcombinations and organizations to create different assemblies suitablefor the functional needs and anatomy of different patients. Also, boneanchors and deflection rods having different force deflectioncharacteristics may be incorporated at different spinal levels inaccordance with the anatomical and functional requirements. Spinalstabilization may be provided at one or more motion segments and in somecases dynamic stabilization may be provided at one or more motionsegments in conjunction with fusion at an adjacent motion segment.

Particular embodiments of stabilization assemblies may incorporatecombinations of the bone anchors, spinal rods, deflection rods,deflectable posts, centering rods, compound rods, offset and coaxialconnectors described herein, and in the related applicationsincorporated by reference, and standard spinal stabilization and/orfusion components, for example screws, pedicle screws, polyaxial screwsand rods—additionally, any of the implantation tools and methodsdescribed herein, and in the related applications incorporated byreference can be used or modified for use with such stabilizationassemblies. It is intended that the scope of the invention be defined bythe claims and their equivalents.

1. A spinal stabilization device comprising: a centering rod having afirst end, a second end and a flexible section connecting the first endand the second end; a ball-rod including at least a partial ball-shapedretainer and a rod; a first bore extending along a longitudinal axis ofthe ball-rod and opening through the ball-shaped retainer opposite therod, wherein the first end of the centering rod is received in the firstbore; a housing; a socket within the housing, wherein the socketpartially encloses the ball-shaped retainer to form a ball-joint; achannel extending from the socket out of the housing, wherein the rod ofthe ball-rod extends through the channel out of the housing; and asecond bore in the housing extending from the socket opposite thechannel, wherein the second end of the centering rod is received in thesecond bore; whereby deflection of the ball-rod bends the flexiblesection of the centering rod and the centering rod exerts a restoringforce to center the rod of the ball-rod within the channel.
 2. The spinestabilization device of claim 1, wherein the channel comprises afrusto-conical surface positioned to contact the rod after apredetermined amount of deflection of the rod from center of thechannel.
 3. The spine stabilization device of claim 1, wherein said rodand said ball-shaped retainer are made in one piece.
 4. The spinestabilization device of claim 1, wherein said housing further comprisesa threaded bone anchor.
 5. The spine stabilization device of claim 1,wherein said housing further comprises a coupling adapted to connect aspinal implant component to the housing.
 6. The spine stabilizationdevice of claim 1, wherein said ball-rod also includes a threaded mountexternal to the housing adapted to connect a spinal implant to theball-rod component.
 7. The spine stabilization device of claim 1,wherein said ball-rod is made of cobalt chrome alloy.
 8. The spinestabilization device of claim 1, wherein said centering rod is made of asuper-elastic metal.
 9. The spine stabilization device of claim 1,wherein said centering rod is made of nitinol.
 10. The spinestabilization device of claim 1, wherein the first bore furthercomprises an enlarged portion within the ball-shaped retainer, whereinthe flexible section of the centering rod is received in the enlargedportion of the first bore.
 11. A spine stabilization device comprising:a first element; a second element; and a self-centering joint connectingthe first element and the second element; wherein said self-centeringjoint is a self-centering ball-joint and wherein the self-centeringball-joint comprises: a housing having a socket; a ball-rod received inthe socket; and a centering rod received at least partially within theball-rod and at least partially within the housing; whereby deflectionof the ball-rod bends the centering rod and the centering rod exerts arestoring force on the ball-rod.
 12. The spine stabilization device ofclaim 11, wherein said self-centering ball-joint comprises a limitsurface positioned to constrain movement of the self-centeringball-joint.
 13. The spine stabilization device of claim 11, wherein thefirst element is a bone anchor.
 14. The spine stabilization device ofclaim 11, wherein the second element is a threaded mount adapted toconnect to a spinal implant component.
 15. A spinal rod comprising; arod having a longitudinal axis; a rod-end comprising an aperture adaptedto be secured to a bone anchor; and a set screw; wherein the rod-end isconnected to the rod by a joint which permits the rod-end to, at leastone of, slide along the longitudinal axis of the rod, and rotate aroundthe longitudinal axis of the rod; and wherein the set screw isconfigured to apply a force to the rod to lock the rod relative to therod-end.
 16. The spinal rod of claim 15, further comprising; a socket inthe rod-end; at least a partial ball positioned within the socket;wherein the aperture is located through the partial ball such that abone anchor secured to the partial ball can pivot and rotate relative tothe socket.
 17. The spinal rod of claim 15, wherein the rod-endcomprises a substantially cylindrical bore in which a first end of therod is received.
 18. The spinal rod of claim 17, wherein: the first endof the rod comprises a circumferential groove; and wherein the rod-endcomprises a pin positioned to engage the circumferential groove toprevent removal of the first-end of the rod from the substantiallycylindrical bore.